Are you using Premiere Elements 11 on Windows or Mac. There is no ChromaKey, Green Screen Key, or Blue Screen Key in Premiere Elements 11 Mac. User of that program are often encourage to try Videomerge. I have suggested consideration of Difference Matte Key.
However if you are using Premiere Elements 11 Windows 7, 8, or 8.1 64 bit, be in the Expert workspace. Go to and click on fx Effects Tab at the bottom of the workspace, select Keying, and then Chroma Key, Green Screen Key, or Blue Screen Key.
If you have Premiere Elements 11 Windows and cannot find those Keying Effects, then I suspect that you are in the Quick workspace and not the Expert workspace. Typically the Expert workspace has more features than the Quick workspace.
Please let us know if any of that worked for you.