I dont have User Mappings. Guess as you said its not included in the Express Edition.

About my attaching the DB I had done it through the normal way, DB > Attach normally and it shows the path to the Db. Then a friend suggested that I type in the name of the DB instead of the path and it worked. Then I got blocked as you said with the Default, but managed to fix that somehow. This was before I got your post and then I confirm it true.

What's the user mapping for? And what if I dont set it off?