If you are buying the iLok version it does not support the addition of model packs. You either purchase the standard version or the Platinum (fully loaded) version.
You can also purchase the "hardware independent" version http://line6.com/store/item/187 That version allows you to install it on up to 4 computers (or line 6 devices) simultaneously, with a lifetime maximum of 10 registered computers or devices. This version will also allow you to fire up POD Farm 2.5 as a stand-alone program for playing or tweaking. The hardware independent version also supports the addition of individual model packs.
There is no maximum computer limit for the iLok, but the only model options are basic and full. You cannot use it in stand-alone mode.