Question on Streetpass....

So I was at a recent festival in downtown Knoxville and my streetpass found 3 other people. I thought I'd be able to do the FIND MII minigame and use them but I can't figure out how... I still have to purchase heroes. Do they have to be online for me to use them in Find Mii or Puzzle swap? How does it work?

Same happened to me today. At least I got people on streetpass and I can't get puzzle pieces or use them on find mii.

I *think* you have to use them immediately. I found that once you go to Mii Plaza and receive the Mii's, you then go to the quest/puzzle board immediately after. Otherwise, if you leave the Mii Plaza application, and then return, you don't have the option to use them anymore.

That's what I gather anyway, as my flatmate as a 3DS and we get streetpasses daily, and I have noticed this.

You have to use them immediately on the day you found them. :(

I guessed it was something like that. Thanks for the help.