Hi,
Just started to review i-doit. So far looks very good. However, I am suffering with the same issue that OP was suffering from:

I can create building locations under the 'Root-Lokation' - this is OK
I cannot associate any room with any building that is created.

I have tried:
1 - selecting the name of the building from the location pop-up (BTW the buttons in the pop-up do not translate to English) the selected location does not pass back to the main entry screen for the Room.

2 - tried typing in the name - again this is not saved

3 - tried typing in the first few characters of a location, and then selecting the location that appeared beneath the field. this caused a very interesting result of placing code into the field and then redirecting me to the page for the building.

I tried this against Chrome, IE and firefox and I get the same result.
I tried the on-line Pro version and this works perfectly although i notice that this is not the same version as the opensource version.

it would be really good to be able to resolve this issue so that our review could continue.