Hi,
First, welcome to the forum

I'd like to know how you installed aMSN, was it using your package manager ? which distribution are you using? ubuntu ? fedora core ? gentoo ? etc... If you used the 'generic installer' provided on our website, make sure you installed the right version, there are two files, one for tcl/tk 8.4, and one for tcl/tk 8.5. That error message basically means that you either didn't compile aMSN (but if you installed a binary package, then it's already installed), OR that you are launching aMSN with a version of tcl/tk that the package was not compiled for.
if you have a debian based system, type 'dpkg -l | grep tcl' to see the list of tcl/tk interpreters installed. For more information, search the forum as help concerning this problem has already been answered many times already.