I stayed here last week. From the underground, follow the signs to Terminal 2. Once you go up the two escalators to arrival level, there is a small sign to cut across the parking lot. Once you're in the parking lot, there's a pretty big sign that says Hilton Garden Inn that can't be missed. Note when you are leaving the hotel the next day, reception/exit is on Level 2, not 0.

If you're flying to the US, give yourself plenty of time. I left the hotel at 8am for my 11:15am flight at T3 and only had like 30min in the lounge before my flight with all the extra security checks.