The mod tools for migrating to any site is broken.
The list stops at Code Review Meta now.
The lack of scrolling is my bug...we don't have enough sites to scroll on our local machines or dev environment, this will be fixed next build.
I'm also adding a "type to search" hint on top the box as well (but you can still scroll like a normal dropdown, if you wish).
As per Nick Craver's answer, this will get fixed in the new build. Until then, you can specify the later sites using the alluded to "type to search" system and typing out the name (partial or full) into the dropdown.
The dropdown is limited to 20 matches, which is what results in your stopped at Code Review for a blank value.