Living on base and using the provided services (commissary, PX, banks) are allowed by the Watchtower Society since you are a military dependant. This applies to all baptized sisters/children who are active publishers and are married/progeny to active duty military personnel. The only exemption is if the job is private baby sitting or private house cleaning. This is considered a private endeavor and isn't within the demographics of working for any entity that exists on base solely for the purpose of supporting the military.
You are studying with a Japanese sister in an English congregation, right?
Active sisters who are military wives may encourage you to get a job on base, but they do not know the reprecussions. However, do this. Ask any elder in your congregation about this and they will site the neutrality issue and will apply restrictions on anyone who isn't baptized (you) and will give an ultimatum to those who already are baptized. The time frame is six months for any baptized Jehovah's Witness to quit working on base if they wish to remain a Jehovah's Witness.
I know. I'm madly familiar with English congregations in Japan.
Alligator Wisdom (aka Brother NOT Exerting Vigorously)