If you're searching for tutorials on applying the perspective extension to some text and having issues, check the following. First, I'm using Inkscape 0.47 on a Mac. The tutorials are advising that I select Path > Object to Path on the text. After that, you're supposed to select the perspective shape, then the text path, then apply the extension.
I keep getting the "The second selected object is a group, not a path." error. My text never shows up as a Path, but rather a group of objects.
If you select the text and then the perspective shape, it should work, even though this is the opposite of what the tutorials are saying.