Where do I go?

I have all of the Guild wars games and expansion. In fact, I just added NF and started a Dervish Character yesterday. My question is this: Where can I find the assassin trainer for a secondary? I looked all over the training grounds but couldn't find any trainers from factions.

I dont think you can chose an Assassin secondary right off the bat, later in the game when you get to the Sunspear Sanctuary you can find a profession changer to get a secondary Assassin profession

Thank you for your help, I've been asking around for that for a few days now. Do you know about how far into the game that is? Ie: half way through, closer to the end?

About 1/3 of the way through NF -- you'll get to the Sunspear Sanctuary not long after moving off the island of Istan to the Elonan mainland.

Assassin is a profession from factions, therefore, there won't be a trainer in NF.
In order to get assassin as your secondary on a dervish, you have to Ascend and/or get the equivalent. The equivalent in NF is completing the quest "Wanted!". After that you have to do Prisoners of War quest that you get from Dunkoro in the Command Post. After you complete the quest, there will be a lot of new NPCs in the Command Post. One of them is the profession changer. You pay 500g and he will change your secondary to anything you want.

Like reddog said, you have to get ascended, to do this complete the quest Wanted! in Nightfall or the mission Naphui Quarter in Factions.

When you finish Naphui Quarter you will go to Senji's Corner and there will be a skilltrainer who can change your secondary.
If you choose to stay in Nightfall and you complete Wanted! I suggest you just go to Great Temple of Balthazar and talk with the profession changer.

initial choices for secondaries from nightfall do not include assassin or ritualist. (factions professions)

NF Secondary Quest

to get assassin 2ndary, complete quest Hunted!

afterwards, u can change into any 2ndary at any Profession Changer

wiki.guildwars.com is an amazing source of information.