One you cannot execute tkprof from the sql promt unless you host out and I forget the excape character for it. But I think you already knew this for you mentioned you tried executing from the command line as well.
Make sure that you are executing from the directory where TKPROF is residing. If you are in a Windows Env you might not have your PAth set correctly to point to your Oracle bin directory.
If on Unix then your environmental variables are probably off.