Restoring a differential backup in SQL server 2005
You haven't shown us your RESTORE DATABASE command, but you appear to have omitted a recovery option, or specified WITH RECOVERY (which is the default). In either case, if you've done that, you cannot apply any further backups.

If you want to apply a series of backups, you need to specify WITH NORECOVERY on all (or all but the last) of the RESTOREs.