Once you open the Metadata Editor you should be able to just press the Add Basic or Add Role buttons to bring up a list of entries to choose from. If you don't see it, try restarting Sigil and opening the Metadata Editor without loading a book. Try moving your windows around in case the lists are covered by another window.
The current version is 0.6.2 - is that what you see under Help->About?
What OS version?