Actually, the most convenient station is Sobudai-Mae on the Odakyu Odawara Line. It's one stop north of Ebina station in the direction of Shinjuku, but only local and semi-rapid trains stop there. It's about a 12 minute walk west from the station to Camp Zama's Gate 4, and from Gate 4 it's about another 10 minutes to the center of the base.
Use the station's North Exit and walk down the road intersecting the main road in front of the station; there's a sign above the road that says "Soubudai" in front of a rainbow-pattern background. Last time I was there, a Century 21 office was on your right as you crossed the street.

Around Sobudai-Mae, you're also likely to see a few Americans who can help you with directions - plus a few businesses in the area probably have English-speakers.

As jib71 and abmj-jr suggested, you may be better off asking if the hotel can arrange transportation for you, especially if the weather is lousy. Some taxis may be authorized to enter the base, but I can't say for sure.

I suggest avoiding Sobudaishita station. It's geographically close to the base, but is a tiny station pretty much in the middle of nowhere - I was once misdirected there by a JR information desk at Tachikawa and found out the hard way. To my knowledge, Camp Zama doesn't have an open gate on that side of the base.

If anyone in your group is credentialed to use military transportation, you may be able to take the (free) bus from Narita terminal 1 to Camp Zama: http://www.usarj.army.mil/informatio...ort/index.aspx

In terms of getting to and from downtown Tokyo, the Odakyu line operates to Shinjuku Station, and you can board express trains either from Ebina (approx. 45 mins to Shinjuku) or Sagami-Ono (35 mins), which is two stations north of Sobudai-Mae. From Ebina, you can go straight to Yokohama Station on the Sotetsu Line in about half an hour.

Odakyu Railways has an English-speaking tourist information center. Detail here: http://www.odakyu.jp/english/center/index.html

Hope you have a good time!