DOn't run 2007, and this is apparently something new with this version, but you can pull up help for any control property by placing the cursor in the property's box, in the property pane, and then click on <F1>.
The concept of 'padding' in other languages has to do with the positioning of text within a 'cell' and I expect the use here is similar, within the confines of the command button. What have you tried using the properties? My guess is that anything entered would have to be in Twips rather than Inches, with 1440 Twips = 1 Inch. Entering 1 or 2 or so, thinking it was inches, would cause no appreciable change in the control's appearance.