To read a personal message, just click on the "You have X personal messages" link at the top of the page, then once the list of messages has come up, click on the message title.

Lots of people leave Mudcat and come back years later. Others drop in every few weeks or months, maybe when something has come up about a particular interest of theirs. There's no rush to do anything in particular here, it's not like other forums where you're expected to devote all your spare time to it and get thrown off for inactivity. It would help to remember your username and password for when you come back, though.