Hi David,
This discussion really belongs at the Artlantis R forums at
http://www.artlantis.com I think?
Yes, only one copy can be active at a time. Initially, no copies are active, so you can serialize any machine. But, you do need a live internet connection to serialize or do remove the license, so as to install it on another machine. This has been the prime complaint about the process on the Artlantis forums ... but personally I haven't found it an inconvenience ... yet.
Piotr's guess of a blocked connection seems the most likely thing if you can't serialize your copy. (Or, it could be a typo ... the "-" sign is part of the license key, so do copy/paste the license code from your registration email.)
Regards,
Karl