There is no 'insert' key with a function similar to that in Windows. If you are looking for the insert/overwrite toggle, you won't find that sort of text entry in Mac apps (and it looks like it's even been eliminated from MS Office 2008 for Mac, though is was supported in MS Office 2004 by clicking a toggle in the status bar at the bottom of a document window). The Mac OS use only 'insert' mode, so to overwrite you must select the text you want to overwrite then start typing.
If you are looking for a keyboard mapping of the insert key for use +in Windows+ (e.g. if you are running Boot Camp), there is no mapped key combo on the MBP internal keyboard, but external keyboards do provide that function. It's fn+return on the Apple Wireless Keyboard and fn+enter on the Apple Ultra-thin (USB) Keyboard.
See these kbase articles:
http://support.apple.com/kb/HT1167
http://support.apple.com/kb/HT1171
http://support.apple.com/kb/HT1216