Changing Profession

I heard that you can change your profession when you are level 20 how do i do that in night fall

You can change it once you Ascend. In NF you ascend by completing the quest "Hunted!". After ascending you can talk to the profession changer in the command post.

In Nightfall you can change your secondary profession in the Command Post. Complete information on requirement are on the wiki.

As noted, the NPC to change your second class is in the Command Post. You will be ascended before you reach there, so that isn't a concern. However, when you first arrive at the Command Post, it will be empty. Althouth it has no monsters, it is not a city, but an explorable area. It does have a LOT of NPCs, including the Profession Changer. To get the NPCs to show up there, you have to complete quests. 3 total, all under Building the Base titles.

http://wiki.guildwars.com/wiki/Build...isoners_of_War

That is the one you will want, as it adds the Profession Changer to the Command Post.

Just to make sure you know it. You can't change your primary profession, but you can change your secondary profession.

As mentioned, you can find the profession changer in the command post, but you can find him in the great temple of balthazar (the one on the battle isles, click the boat on the map to get there), without having to do a quest to get him there. However, you must complete the quest Hunted! before you can make use of his services.

then how do i get avatar of balthar i got it in pvp and my MOX got the skill but i didnt

You have it unlocked with balthazar faction. That means PvP characters and all heroes across the account can use the skill. For a PvE character to learn it you have to do one of two things: capture it with a signet of capture from the boss who has it, or learn it with an elite dervish tome after you unlock it.

To get an elite skill (such as Avatar of Balth) on a PVE character that character must either:

a) Use a capture signet to capture the shill from the boss that has it, once said boss is dead
or
b) Have the skill unlocked on your account (which you have) and for your PVE character to use a suitable elite tome (ie elite dervish tome) to learn the skill

Best place to check the skill you need if you havent gotten a elite or regular tome to use is here
http://wiki.guildwars.com/wiki/Skill
This place will show where the NPC is located or boss/mob to get the required skill from