I also had trouble installing plTCL, and the problem turned out to be that the PGLIB environment variable wasn't getting exported since I wasn't doing a full
su - into the postgres user. However, this problem generated a different error message than you're getting, so I'm not sure how relevant this is to your situation.
Anyway, you might make certain that you're actually the proper user with all the proper env vars set properly while you try to load the lang. Good luck!
