FWIW, the AMP GUI's mod ID gadget didn't take input at all. It was there, but it wouldn't take focus. And it was still doing this in 1.7 nightly from about a week ago.
What I did to get mods into SE was download the sandbox.sbc file, edit in the mod ID, and re-upload the file back using filemanager from the AMP GUI. On startup, Space Engineers downloads the mods that were called for without you having to do anything to install files.
It would be really nice to have that GUI in AMP work though. I worry about manually edited stuff like this getting overwritten by the GUI when it refreshes, having been "blank" the whole time.
FWIW, the manual edit I mentioned works fine to get mods in, but the AMP GUI doesn't also seem to read back the config, because the AMP interface never recognized the mod info was manually added in the Sandbox.sbc file.
If you go this route, you'll want to look for the
<mods></mods> line and add in your mod ID like this:
You can get the numeric portion from the URL in Steam's interface when you're looking at the mod.