First off I need to tell you that you do have the correct address. That is the address to Leonard's agent.
I found one other site that I thought was going to get you a different way to contact him, but it did not.
I found: http://www.theofficialleonardnimoyfanclub.com/
It has a section for fan mail, and if you read it, you will understand they do not know of another way to reach him directly either. Not to mention, I believe this fan club is based in England, and I was not sure, but it is possible that Mr. Nimoy may have a home over there as well.
Don't take it personal...
A lot of times, depending on the celebrity, how private they are, or how interactions with fans has gone for them over the years, this may be the way they have chosen to 'deal' with their fame.
Sometimes this may be the ONLY way.
The address this fansite has given differs from the one you found, by just a little bit. So be sure to check it by calling the Agency first, and verifying with them. The address this fan club has is:
Leonard Nimoy Fan Mail
c/o The Gersh Agency
232 N. Cannon Drive
Beverly Hills, CA. 90210 USA
I hope this helps you out. Oh and be sure to ask him, "What ever became of 'Primortals'?"