The guides for the N900 sort of gloss over the process a little bit. They are not wrong, but knowing more than which buttons to press and in what order is perhaps required knowledge. It is significantly easier if the network has clients on it, but having clients are not always required.

If I might suggest you have a read through this particular page, you'll be in a better position to access an AP using WEP without clients.

You'll need to be familiar with working in an x-term - I don't know if the GUI for doing this exposes the right aircrack-ng switches - if it does, cool, if not, the shell is a good place to get things done too.