Welcome to Splinterlands @harpun.

It depends on the package you bought. Starter pack? Booster pack? Or Untamed pack. If it is the Starter pack, all you need to do is login to splinterlands and start playing with the cards you. The cards from the starter pack are not yours but they allow you play the gain.

If you bought the booster or untamed pack, the cards you get are yours but you can't play the game until you buy a starter pack which will activate your account to receive all benefits.

I suggest you contact them on their discord channel to get better clarification.