Rene, go out the Wakamatsu kawada station through the Kawada guchi exit. Turn left (or west) and walk for five minutes. You will come upon a gas station on your left. Take the first right after the gas station. There will be a sign in kanji saying "Aikikai" at the intersection. Walk along that road for a few hundred feet. Hombu dojo will be on your left.
Rene, Raul's explanation is clear and easy to follow. A few tips: the subway line is called Toei Oedo Sen (Line). When you get off at Wakamatsu Kawada station there are TWO exits. If you go out the other exit you will have to turn RIGHT. The direction toward Hombu is slightly downhill when leaving the station. This is always a help when you are directionally challenged like I am! I recommend making a dry run from your hotel to Hombu Dojo once, especially if you are planning on trying to get to the morning class. It is not good form to come late to class, and it's also not fun getting there too early either. It helps to actually make the trip to estimate travel time, without having to stress out about not getting there in time. Also: remember to go in through the door that leads in to the men's changing room-straight ahead when you go up the stairs. The curtained entrance on the right hand side is for the Sensei (and female practitioners, so they don't go through the changing room!)