New video card not working

Hi to start this off i wanna say that this is my first time trying to customize a pc. Recently i purchased the p6280t.

http://www.shopping.hp.com/webapp/shopping/computer_can......

the specs are listed there and i have all the basic stuff. Integrated graphics, windows 7 home premium edition, Q300 etc.
I bought a radeon HD 4670 1GB DDR3 and a Corsair 400w psu. I've installed the psu successfully it was very easy with no problems at all. Now the problem that i have is with the video card. When i try to slide in the video card it wont lock in i dont hear that popping clicking sound it makes when u put in a card. Isn't it supposed to? Its very loose and I believe it will fall off if it wasnt for monitor port that sticks out of my computer in the back. And i know its the right slot! The PCIe x16. I've been looking all over google and couldnt find anything relating to my problem..
this is my video card btw http://www.newegg.com/Product/Product.aspx?Item=N82E168... does it matter that it takes up 2 slots? I tried it on my other HP computer and it doesnt fit in there either. Another thing my installation guide says to secure the mounting bracket of the graphics card to the chassis with a screw ....the problem to that is there IS NO SLOT on the chassis to do this. its not designed like this for example http://www.youtube.com/watch?v=RPB_uY00_yg. jeez this is pissing me off so much.

Help anyone?