Stepkdramafan226 - Do you mean "bookmarking" via your browser, or are you talking about notifications of new replies?
For bookmarking via your browser, it would help if you let us know which one you're using. I use the latest version of Mozilla Firefox, which has a star icon on the top-right tool bar for bookmarking pages/threads.
For notifications of new replies (either on-site or in email), sadly, the MDL forums do not have this feature. It's such a shame, because it seriously inhibits community discussion/interaction. I can't count the amount of times I've forgotten about a thread, only to see it pop up again months later, with a bunch of replies I've missed. I've started bookmarking (via my browser) certain threads I want to keep an eye on, but honestly, it's a PITA and I still tend to forget to check those bookmarks. :(