Thank you Cipes, but, as I said, the dimension class is visible, and the show/snap/modify others is active (I tried also with show/snap others, and it didn't work).
Yesterday I was working in my laptop, and today I tried again at my desktop and I managed to make the chain dimensions not to disappear (although I have no idea of what I did differently). Still, the trouble of being unable to modify the intern walls position by modifying the dimension value goes on. I don't know what I did or what to do, and I'm getting tired of trying to do exactly what the tutorial says and not getting the same result, without any idea of what or why it's not working...
I think all tutorials should have a "common mistakes" section. They just tell you what to do when things go all right, but none of them teaches you what to do when the step-by-step doesn't work.
Update: I picked other endpoint of the intern wall, further from the dimensiion, and finally was able to reposition it. But that's not what I was supposed to do. Why did it work in one situation and did not in the other?
Sorry if I'm bugging you guys, but I'm a SketchUp and Autocad user, and migrating to a BIM software is being very difficult to me.