Cannot get Multi-DB to connect

(I realise the plugin section is wrong but Multi-DB doesn't appear in the options dropdown!)

I'm going through pretty much the same grief as most users seem to with Multi-DB because the instructions for this are so ambiguous.

At present my move-blogs.php is throwing up:-

"Database Error: Access denied for user ''xxxxx'@'%' to database 'zzzzz_c'

(data obfuscated)

Well, user xxxxx'@'% doesn't exist so that's got me stumped! The stopping at database _c seems to correlate with a query in another Multi-DB thread by svteg.

I have an existing database with existing tables, and I've set up a blank _global one as well, but no dice. Do we use IPs or specific localhost:port addresses? Why a 3 block IP at add_dc_ip and not 4 (or is it 4 anyway - comments show 3, the instructions show 4). The comment in another thread:-

"Line 25: add_global_table('restrell');
Do you have a table in your global database called wp_restrell? If not then that line shouldn't be there."

Well, I thought the instruction was to add the table so it shouldn't be there anyway!

I'm really and truly lost. Can something please give a clear and unambiguous set of visual instructions for what should be a relatively simple setup. It's the only element of WPMU DEV that's truly been beyond me at present.