Hiya! Honestly, I wouldn't recommend using Advance Text at all - it's buggy and over a decace old at this point. Advance Text might not have the offsets of those specific text strings stored, which could be why you can't find them to edit them.
It's beneficial to learn
scripting with XSE for FireRed binary hacking in any case, but even if all you wanted to do was change sign and Mart data, it would only take you a few minutes to create your own scripts. A-Text could cause you all kinds of problems by not repointing correctly/overwriting things, but using free space to create your own scripts would stop that from happening. Specifically, just to get you started, I'd check out this page for simple text/
signposts and this page for
Marts (scroll right to the bottom). Hope this helps!