Moving cursor in Source Data - Values text field

I have searched the Excel help, Microsoft web site, your site, and the web to no avail. This seems like such a simple thing to want to do...

I want to change the data that's displayed in a Chart, so I right click somewhere that offers a Source Data... option and click on it to bring up the Source Data dialog. I place my cursor in the Values text field to manually change the data to display. I try to move the cursor using the arrow keys and instead the current selection moves in a worksheet and the address of that selection is inserted into the Values text field where my cursor is.

I don't want the blasted selection to move in the worksheet, I just want the cursor to move in the Values text box.

So far, the only thing that seems to come close to working is to turn Scroll Lock on and then hold down Shift while hitting the arrow keys. Of course, this causes text to be selected in the Values text field so that once I get the cursor to the right place I have to click with my mouse to deselect so I can enter what I want.

Help!