I am fairly new to AE but have been given the task of creating new Lower 3rds for our videos. This is what I am wanting. on the left hand side I want it to come from the diamond scrolling to the right basically coming from the top and bottom points of the diamond and then when it move back the same way. I hope that kind of explains what I want to see it do. Basically in the masked portion of the screenshot I don't want the grey area and want the mask to stay stationary.
I'm still a bit confused. Are you just trying to mask out the gray part of the shape layer that the mask is covering in the screenshot? What are your mask settings? Another visual example or test-render might be helpful here.
I don't quite understand what you are saying, eitehr. If you mean you want the crners of the rhomb to be the edge of the wipe, then well, simply use a wipe. Your'e making this far too complicated. Create the cover in the pre-comp, then move it as a whole layer in your parent comp. That makes two keyframes for it coming in, two for it to zap back after a while. No masks, no futzing around.
I'm thinking you want to have the the text and the arrow move out with the background like they are appearing from the diamond with the KC in it. I'd create a new comp with just the text and the gray background and the red >. Drop that composition in your master comp, rotate a square solid 45º and place it above the nested comp but below the Diamond KC. Set the solit as a track matte for the gray solid and animate the position.
There you go, you're done. Took me less than 2 minutes to produce this screen shot.