Either Enter Time Machine while the Library is displayed in the Finder, or while in Time Machine, type cmd-shift-g and enter ~/Library.
If you are digging down a longer path, you don't have to type out the full name. Just type enough characters that it should be unique and hit tab. It will fill out the rest if it can identify that folder. If it doesn't, add a few more characters.
PS ~/ is short for your home folder.
If you prefer to make your user library permanently visible, use the Terminal command found below. It does work with Mountain Lion:
You might want to bookmark the command. I had to use it again after I installed 10.8.2. I have also been informed that if you drag the user library to Finder it will remain visible.