Hi Mark,
To change the text of the "Add to Cart" button on the product details page open the following resx file:
ProductDetails.ascx.resx
Located in the folder desktopmodules\smith.productdetail\app_localresources\
Then change the value of key titled "ibAddtoCart.Text". You probably just changed the wrong value.since there are several rows with similar names for all the related products add to cart buttons.