That Gmail IMAP addon doesn't install, as it is incompatible with SeaMonkey. Gmail IMAP will set up manually, however, so do not know why addon is needed.
Hmmm.... okay. Gotcha. Maybe the instructions could be edited slightly, to include people like me. Hard to understand that, from the way it is presented visually. Just saying. I'd edit it, but it won't let me. Thanks.