I saw this once when I installed different sets of GNU freeware from the SGI site. Irix has a version of tar (in /sbin, I believe) that doesn't support the same set of command line options (-z for gzip tars, -j for bzip2 tars) that GNU tar does. I believe I had an incompatible version of glibc installed for the installed version of GNU tar, and the GNU /usr/freeware/bin directory was earlier than /sbin in the path, which triggered the 'tar is bad' error.
There are several options: make sure GNU tar and glibc are in sync; force the use of Irix tar (since you're using the software manager) by removing any references to other versions of tar from the path; or manually extracting the tardist that you're trying to install into a working directory with gunzip and some form of working tar, and then pointing the software manager to the working directory instead of the tardist.
Hope this helps.