The Acceptable Value area is where you'd configure a set of values to which Album Artist Sort Order would be restricted. So you'd want to leave this area of the field definition blank. In other words, any value is acceptable.
There is probably no reason to add a value to the search field, as the Artist fields will probably cover any searches. But it is safe to leave it - if so, consider making it an abbreviation so you can enter it in Search more quickly.
Once you've made the changes, then you can select the file(s), and perform Library Tools > Update Library (from tags) to re-read the tags and have this field populated with the file's tags. Check this by looking at the field in the Tag Action Window. You'll probably need to add the field in the TAW's view - right click the view to see the options.
More info about the Manage Fields dialog:
http://wiki.jriver.com/index.php/File_Properties_(tags)