Very Mini Bundle Train

Plural of DLC would still be DLC, since you base pluralization on the component words.
Eg "I bought this downloadable content. I made multiple purchases of downloadable content." and not "I bought these downloadable contents", which would actually have a different meaning altogether.

In your case, it'd be "I stupidly activated one of the DLC packs."
But as it's a commonly understood acronym, and pluralization doesn't affect meaning, and we're not writing a formal English paper, pretty sure not too many people'd pay it any mind :P

And for what it's worth, I did exactly the same thing- activated the DLC, then noticed the season pass [whereas, had I noticed it later, I'd have held off on activating the DLC, at least for a bit.]