Text isn't limited to text boxes. You can click most shapes in Microsoft PowerPoint and start typing text. This is because most (though not all) shapes have a text frame. To work with text in shapes programmatically, you start with the text frame. Here is a simple macro:

Sub AddSomeText()
	' work with the currently selected shape
	With ActiveWindow.Selection.ShapeRange
		' the shape's "text container" is the TextFrame
		With .TextFrame
			' unless you specify otherwise, .TextRange refers to
			' all of the text in the text frame
			With .TextRange
				' add some text
				.Text = "You can add text to shapes"
			End With
		End With
	End With
 
	' or somewhat more compactly
	With ActiveWindow.Selection.ShapeRange
		TextFrame.TextRange.Text = "This works too"
	End With
   
End Sub

Most of the example macros here start with ActiveWindow.Selection.ShapeRange, which you can't replicate in AppleScript. This situation will likely surface often. In Microsoft Office 2004, you paste the GetSelectedShape() handler at the bottom of every script that needs it. The handler does not appear every time it is needed in this topic, but it is almost always called, so you always need it.

tell application "Microsoft PowerPoint"
	--get the selected shape via the handler
	set selectedShape to my GetSelectedShape() --paste it in below!
     
	tell selectedShape
		tell its text frame --needs 'its'!
			tell its text range --needs 'its'
				--unless noted, refers to all 
				--the text in the text frame
				set content to "You can add text to shapes."
			end tell
		end tell
	end tell
     
	--or, more compactly
	set content of text range of text frame of selectedShape to ¬
		"This works too!"
     
end tell

The first time through, leave out the final line if you want to see the first text change — it is replaced by the second one so quickly that you won't spot it. Note that using nested tell blocks, which correspond to With blocks in Visual Basic for Applications (VBA), is useful if there are going to be more commands directed at any of the intermediate targets. Otherwise, for a single command like this one, it's simpler to use the more compact of version. You can use any combination of tell and of you like. It needn't be all or nothing.

This is a very useful example because it contains two properties — text frame and text range — that are the same term (keywords) both for the property of their containing object and for the class. As a result, you absolutely have to use its in tell blocks and in whose (where its) clauses. You will notice that with two of them here, if you omit the first its for text frame, the script won't even compile for text range.

Sub TextCaveats()
   
	With ActiveWindow.Selection.ShapeRange

		On Error Resume Next
       
		' does it have a text frame?
		If .HasTextFrame Then
			' does the text frame have any text?
			' see if there's any there
			' before trying to alter it
			If .TextFrame.HasText Then
				.TextFrame.TextRange.Text = "And here you are, safely"
			End If
		End If
	End With
 
End Sub
       

tell application "Microsoft PowerPoint"      
	--get the selected shape via the handler      
	set selectedShape to my GetSelectedShape() 
		--paste it in below!      
	try --to avoid error messages if shape 
		--has no text frame or no text           
		tell selectedShape               
			if has text frame then --does it have one?                    
				--if so, does the text frame 
				--have any text?                    
				if has text of its text frame then 
					--needs 'its'                         
					set content of text range ¬
						of its text frame to ¬                             
						"And here you are safely."                    
				end if               
			end if           
		end tell      
	end try 
end tell

tell application "Microsoft PowerPoint"
	--get the selected shape via the handler
	set selectedShape to my GetSelectedShape() --paste it in below!
	set check to my IsSafeToTouch(selectedShape)
	if check then
		set content of text range of selectedShape's text frame to ¬
			"Different text depending on context."
	end if
end tell
 
on IsSafeToTouch(selectedShape)
	tell application "Microsoft PowerPoint"
		try --to avoid error messages if shape has no 
			--text frame or no text
			tell selectedShape
				if has text frame then 
					--does it have one?
					--if so, does the text frame 
					--have any text?
					if has text of its text frame then 
						--needs 'its'
						return true
					else
						return false
					end if
				else
					return false
				end if
			end tell
		on error
			return false
		end try
	end tell
end IsSafeToTouch