loading new assets from client app (How to?)

I have a quick question for savy tech programmers out there. I need to be able to add additional content to a game which is installed on the users computer via a unity installer. In other words, it would be like DLC content, but this content would just appear for the user as additional unlockable items when they logged online with the game client. The game itself would contain a login, so each user would be identified in a database which would run on our servers. Ideally I would like this solution to work on users who have installed both MAC and PC version of the client package.

Are there any hints in the unity user manual which would explain how I could accomplish this?