I’m with Bill, it’s easier to separate the layers. My steps:
Create a new file, white background (that’s what I did, but the background doesn’t matter for this one).
Create the “shape” on its own layer using the rounded rectangle tool (I set the color to black).
Create the type on the next layer (black type).
Duplicate the shape and move the duplicate above the type layer. Change color to white. Right click the top shape layer in the layers palette and choose “Create Clipping mask.” (or option-click between text and top shape layers).
You can also link the top and bottom shape layers so you can move both around.

Save early. Save often.
Jonathan Ziegler
http://www.electrictiger.com
520-360-8293