Its no use. I tried clean reinstall, and nothing changed.
Any idea, why this is happening? Maybe some permission issue?
What files are requesting for this drop-down list? Where I need to look?
Looks like all this case was caused by high restrictions of chmod.
For import, I applied chmod 777 to all files and directories, and, voila! Now I can select system to import from.
Later, I`ll make some tests about setting "read" ability to "/library/XenForo/Importer" directory.