Signature is about as close to the airport as you can get. It's pretty much a straight shot.

From the airport, or from the car rental center?

Swenson to Tropicana is correct.

As you pull out of the airport, follow the signs onto Swenson going North. Your first main light is Tropicana, go left.

If in doubt follow the taxis. It's very easy though.

Note that the entrance to the Signaure is off Harmon, not Tropicana. Here's a map assuming you are coming from the car rental center, not the airport.

http://www.gmap-pedometer.com/?r=2989095

That's probably the best way, it swings you off the strip to avoid some traffic. As an alternative, you could just stay on the Strip all the way to Harmon.