User blog comment:GeneStarwind/Monster Appreciation Day!/@comment-168.169.237.135-20110531180028/@comment-1696682-20110531213714

You gain access to the Long Sword in Monster Hunter Tri after you beat the Gobul Urgent Quest. If the Argosy was in port when you do, you'll have to wait for it to come back again before you can build your first one.

You can build them from the start when Online, however, you need Baggi parts for the first Long Sword, which you can't do 'till you hit the 2* quest and can start hunting them down.