Apparently the address changes regularly. The address I used isn't "correct" anymore.
Look here:http://www.fanmail.biz/99609.html(and below, without the dots in the name)Via Venue Address:G.rac.e P.ar.k
"Hawaii Five-0"
Eye Productions
605 Kapiolani Blvd
Suite 2
Honolulu, HI 96813-5195
USA
Fan Mail Address:G.rac.e P.ar.k
The Characters Talent Agency Ltd.
#200 - 1505 West 2nd Avenue
Vancouver, BC V6H 3Y4
Canada
I used a Honolulu address, very similar to the first one. It's usual to "find" several addresses: 1 for the studio, 1 for the agency, for example. This can explain how it can take so long till it gets forwarded to them. (and then they must find the time to lower their fan-mail piles

... )
In 2007, when I wrote to her the first time, I had used the Vancouver Film Studios address. (doesn't work anymore, of course - it was for BSG)
Be aware it can take 1 month or two years, depending on I don't know what.

Luck, I guess.