Which version is the 727 Base Pack? Your 727 Freighter Expansion Model is v2.4 (seeing as you just bought it). It won't work properly if the base pack is not v2.4.

The base pack needs to be csp721_2400.exe if it is anything else (i.e. csp721_2300.exe) then you need to download the newest version.

To get the latest version of the 727 Base Pack, Check-in to Your Profile > Product (727-100 Base Pack) > Extended Download Service > Get New Links

Hope this helps