NUchem (could be a pseudonym) suggested a screencast about installing GAMESS. In my case this will have to be installation on a Mac, and the installation on a Windows or Linux computer will be quite different.
If others want to volunteer similar screencasts of Windows or Linux do leave a comment on the blog. I'd be happy to post them or provide a link.
I use GAMESSQ to run GAMESS. It is also possible to run GAMESS from the command line, but I find GAMESSQ much more convenient, so this is what I suggest you use on a mac.