I wish I could answer your question directly, but I was not home when they put our carpet in on Monday. I can offer some other advice. You don't need a subfloor. If you have the headroom or had previous water problems, it is a good idea. My carpet and pad are directly on the concrete. The key is the pad. I have an 8 lb. pad - very dense. You can't tell its on concrete. It actually feels better than the upstairs which is over a subfloor, but with a less dense pad.
One problem with attaching the carpet strips is nailing thru the concrete. If you don't get a satisfactory answer, call an installer. Or better yet, let them do it. I did. It was worth the extra $$ to me.