If I understood you correctly, the text is not a part of the background video.
A button "consists" of 3 things
- underlying text / image which is a part of the video stream
- overlying text / image which is usually identical to the underlying text / image, but it can be also just e.g. an arrow - that's the thing you see when you move your cursor to the button
- information in IFO files
What you probably need to do is to right-click on the video background, and select render motion. DVDLabPro will make for you a new video stream which will consist of the original video stream + the added text, and will automatically replace your menu background with the new video.
Hope it's clear, my "technical english" sucks