You should get simPE .73, which is linked from the news article on the home page. It needs to be extracted into its own folder. (NOT the folder you already have SimPE in!! It needs a clean installation.) but you can edit the properties of your current simpe desktop shortcut to point to the simpe.exe in that folder, and your file associations will have already been set by the old Simpe installation.
If you still see the error, it is probably having trouble recognizing a multipack that you have. I forgot what you need to do, but I think that going into the file table and manually pointing to the directory for each part of it might solve the issue.