Copied over from msdn forums:
1) Go to restore database
2) Select the database that you want to back up to
3) Locate the backup file on disk. You may have to put it into the MSSQL Server -> MSSQL.1-> MSSQL -> Backup Folder. It must be a .bak file.
4) Select the back that want to restore from the available backups.
5) Go to the top left "options" property and when you do that select "overrite existing database".
6) Now make sure that the path to the files on database to be restored are correct in this same dialog view. Look at the paths to the database file and the log file and make damn sure that they are the correct ones for the database to be restored. The problem here is that those paths are going to be for the filesystem that the backup came from, not the one you are goning to put the restore onto.
Just to add to that, if you are restoring a database to a new server that does not have the database already in it (to restore to), create a sham database with the same name, then restore to it with the overwrite settings mentioned above.