Go garage->Tuning->Tyres.
When you buy them, they ask you, if they should put them on, so it could be that they already are on.
I had this problem too.
I bought a set of dirt and snow tyres in advance, but didn't have them fitter right away.
When I eventually went to do the rally it wouldn't let me, and I couldn't find how to fit them, so I had to buy them again and choose to fit them now.
I'm having this same issue. I bought dirt and snow tires but chose to not install them. It said it went to my "items" or something, but now I can't find them to install them.
Edit:
Oh.. duh. Go to the items button (to the left of your garage button) and you can install them from there.