Here you can find the documentation for Mantis, especially the FAQ.
Q: I want a nice quick way of linking to other bugs.
A: Just prefix a bug id with the # sign. So if you type #742 then it will automatically create a link to bug #742.
Q: On the "View Issues" page, I just want to see the Squeak bugs, not the bugs for OpenCroquet or Squeakland, etc. These are listed in the "Category" column, and filtering by Category doesn't work.
A: Squeak/OpenCroquet/Squeakland/etc are actually "Projects" not Categories. Go to the menu in the upper-right corner and select the Squeak project. Then you can still filter by Category to see only the Morphic or VM bugs, for example.