If I remember correctly the wording at the time was that the DLC would be made available for PC when CM had figured out how to handle delivery. Unfortunately they couldn't solve (or be botherd IMO) so no DLC for PC.

I still think you should get the game, it is quite fun.

OK thanks. I already bought this game here on GOG but couldn't figure out yet whether the DLCs are included or not.

I just found a thread on Steam forums where a guy got files from the DLCs from the Mac version of the game (the Mac version comes with the DLCs as seen here: https://itunes.apple.com/en/app/grid/id594773178?mt=12) and added them to the game's folder on Windows.
So if you get the files that the Mac version has and are not present in the Windows version (except for runnable binaries and libraries, of course) you should have access to the new content.

I haven't tested it (I actually don't have the game yet) but some people on the Steam forums claim it works.
Hope GOG releases the Mac version so that everyone here can try it.

http://www.moddb.com/mods/8-ball-prestige-packs-dlc-pc/downloads/8-ball-prestige-packs-dlc-pc
Here you can find the DLC download ;)

Nice find. Know if this works with the GOG install?

Yes it does work. Tested it myself ;)

Good find! They not just simply work for the GOG version, the integration is perfect: even their respective icons show up in the main menu under Extras, like they were meant to.

Sadly the game is no longer sold here or elsewhere because of the expired licenses (which is a damn shame as otherwise it's a gem in the racing genre), but nonetheless this thread deserves a bump for those fortunate enough to have bought it already.

(And I also think that the DLC files should still be added as goodies as they are official content from a published release.)

I installed the DLC and it comes up in the menu, but I cannot select anything side it.
Is there any prerequiste to unlock them?

It's normal that you cannot select them, the icons there are only meant to signal that the DLCs were installed properly and the game recognizes them. All their contents are automatically unlocked if you can see them under Extras, but you cannot use them in GRID World (the champaign), only in custom games and multiplayer. They are still a nice thing to have.