In addition to what Joe said, be sure you tape ALL the sides down COMPLETELY leaving no 'edge' exposed. Use duct tape, and 'clean' the floor before you put the tape down so the 'dust' doesn't prevent the tape from sticking. After a couple of days, if you just have minor condensation, you should be OK to put carpet down. Just don't do anything, like lay plastic down first, before you install the carpet. In doing so, you're 'trapped' the moisture and you'll have all kinds of mold and mildew problems. Let the moisture rise up through the padding and carpet. If the humidity is high, get yourself a dehumidifier. It's a very common practice for basements.