Can someone please help me with IK

Hi everyone

below i have links to 2 images and 1 to a file that hopefully one of you can figure out for me if the images don't help.

Ok so here is my problem, I have IK set up on this arm thing and the elbow of the joint where the red line is I want that to slide up and down but when I set up IK I cant move the elbow up and down. now thats pretty much it if anyone can help me figure this out it would make me real happy.

i`m not so good with the words so if any of you didn't understand what i need just ask and I will try and explain better :) thanks

http://www.meyitzo.co.uk/open.jpg
http://www.meyitzo.co.uk/closed.jpg

http://www.meyitzo.co.uk/help.rar

mark