The page you link to is to download tcl/tk8.5 (which is still in beta by the way).
I'm on a linux system, but I got that same message you mention once, and I found out it was because I had compiles amsn with 8.5 (like you did) but the "default" (so to speak) version instaleld was still 8.4, and that's what caused the problem.
Try running amsn with wish8.5, if freebsd works similar to what I'm used to this command will be it:
wish8.5 ./amsn