Editing of Revtex AIP Style not possible

Yes, your suggestion solves the problem. Now I can see the source,

and I can modify it.

Really really, thank you a lot for helping me!

It is only half of the working state. I would still check if you have the source.ts style file that is in the folder Styles, because setting that as style file (through the menu Document->Style) you would be able to see the source and the comments.

I see,
So, after reading your comment I decided to proceed with the installation of the newest version of TeXmacs. The link was provided in one of the previous comments by @pjoyez. I decided to do this because I also found out that my problem with the first-page footer was because of a bug in version 2.1.1. that I was running.
From this image, I believe you see that the problem is now completely solved.

NOTE: Please note that my original installation [version 2.1.1.] came from the TeXmacs website. That is the version that is downloaded by default. It seems that there is also 2.1.2. version but it is not directly linked from the download page so that would certainly be good to correct.