@bloggy2013 and @Deleyna You two are spinning around making yourselves dizzy. I can't follow all the things you are trying.
Not the "plugin is screwy", and what are you trying to change IDs for? There's no need for that. Follow the instructions. (If you used the wrong Site ID defining a mapping, delete it and create a new one.)
For Everyone:
My instructions have a section on how to correct the settings after trying everything under the sun (how do you think I wrote my instructions!) Server Admins "know" too much, that's how years ago I got jumbled, but it's really straightforward.
Yes, you do need to clear caches. Turn off WordPress cache plugins until you have domain mapping working. Turn off browser cache, or clear it often. In Firefox I use the Web Developer Extension, has an option to disable browser cache.
Minimal Security:
Do yourself a favor, and Never use wp_ as your table prefix. "Every" SQL Injection script written assumes you used that. (Don't use wp3_ or wp4_ or the domain name, either). If your site is set up, use a plugin like iThemes Security to change it!
Do yourself a favor, and Never use admin as your login name. Use iThemes Security to change it if you did. Have one login name for writing/editing, have another for administration; use nicknames for every user so the login name is never displayed on the site. Then always limit login attempts (iThemes Security).
Back to Basics:
1) Is WordPress installed in the root folder of your account? (Or, in the root folder of a subdomain of your account?) Domain Mapping can not work with WordPress installed in a subfolder, not a limitation of WordPress or the Domain Mapping plugin, but of DNS on shared hosting.
2) Before you installed the Domain Mapping .htaccess rules, does Wildcard DNS work? Browse to http://asdf. yourdomain.com/test.html (where there is no subdomain asdf, and where test.html is some "Hello There" minimal HTML page). Does it display?
Don't bother trying to install or test the Domain Mapping plugin until Wildcard DNS is working.
3) Have you removed all .htaccess rules that might affect domain mapping? Have you removed any subdomains you might have made for your mapped domains? The only .htaccess rules specific to domain mapping that you should have are the ones WordPress tells you to use.
Exception: You should redirect mappeddomain.com to www. mappeddomain.com (or the reverse, whichever you are using) in .htaccess [please forgive the space in there, it's just to make it not display as a link...]
4) Have you reset the options in /wp-admin/network/settings.php?page=dm_admin_page since switching to the Development version of the domain mapping plugin? The settings must be reset from the prior version.
What works on my setup (development revision 940562 ver 0.5.4.4) is :
1a Redirect administration pages
not 1b
1c Map links in /wp-admin/
2 Permanent redirect
3 User domain mapping page
not 4
5) Delete all your domain mappings.
6) Go through all the Detailed Configuration in http://lcblog.lernerconsult.com/2012-wordpress-3-multi-site-domain-mapping/ to clean out errors you probably put in while trying everything under the sun. (Guess how I that....)
7) Re-create your domain mappings.