This plugin has a number of bugs that are just plain dumb. Here are the fixes:
The search function just plain doesn't work. Why? Because it expects you to put the query in SQL syntax!!! That is, you can't search for example and expect http://www.example.com and http://www.example.net to come up; you have to search for %example% instead! Since partial searches are obviously what you want in this function, look in domain_mapping.php for the line containing
WHERE domain LIKE %s and change the text after it from
'%'.$domain.'%'. From then on, search will work.
The list of mapped domains is limited to the first twenty or so. That would be OK if there were a paging system, but there isn't one. So look in domain_mapping.php for the text
LIMIT 0,20 and delete it. After that, it will display all the lines!
Function dm_edit_domain() gives a warning about an uninitialised object. To fix it, insert a line above
$row->blog_id = ''; that says
$row = new stdClass();. The warning is harmless but careless. Plugins should never emit warnings - they're a sign that the programmer hasn't tested their work.