right, this should have been pretty straight forward but just wasn't!
i followed the tutorial by otto but still couldn't get it working and was left hanging until just now when i had a burst of inspiration. i'm using the subdirectory method of multisite so i don't know if this will work with the subdomain set up too...
this is not simple stuff and if you're even slightly worried about messing about with servers and "behind the curtain" web stuff, find someone qualified/skilled to do it for you!
here's how i did it (i use hostgator and have cpanel on my hosting):
-install the plugin as per instructions
-get the ip address of your server your site is on (usually available in your hosting cpanel somewhere but there are websites out there where you can get that info by entering the URL) and put it in the A RECORD box of the plugins settings page. putting the ip address here doesn't "do" anything more than tell your users what it is.
NOW YOU AND/OR YOUR SITES USERS MUST GO TO YOUR DOMAIN NAME PROVIDER (i used LCN.com but a lot of people use godaddy.com) and find the section that lets you manage the domain (eg mysecondsite.com) you want to map.
-go to the section that allows you to manage DNS settings.
-in the A RECORD section paste in the ip address you found earlier and save your changes.
***(THIS SECTION REQUIRES YOU TO HAVE ACCESS TO THE SERVER SO YOU CAN PARK DOMAINS AND MAKE CHANGES TO THEM. if you're a network admin running your own network then you've probably got access and will have to do this step for your users, if you're not then you need to find out who does have access to help you do the next step)***
NOW GO TO YOUR HOSTING CPANEL (or equivalent) and go to the PARKED DOMAINS section.
-park the domain (eg mysecondsite.com) you want to map
-find the "manage parked domain redirect" option for your newly parked domain and then enter the full domain of your MAIN SITE + the sub-blog that you're trying to map the domain to eg: mymainsite.com/subsite
save your changes.
go into the dashboard of the subsite you are trying to map the domain to and in the tools menu find the domains option.
from here you'll need the site ID number (the number that corresponds to the order in which new sites are created. if you're running the whole network you should be able to find this by going in as a network admin and reviewing the list of sites on the network).
enter the site ID and then the domain name you're mapping, eg mysecondsite.com. if it's the only one that'll be linking to the subsite then check the primary box and click save. (if it's not then repeat this process with each one and then when you're done select which one is to be the primary and click save).
now for the acid test: open a new tab in your browser and type in the web site name you just mapped and you should go to your subsite but the domain name of choice in the address bar.
hopefully this has worked for you. if there are any issues they may be simply down to spelling! :D always check that you're consistent with domain name spellings as you go, especially if they're complex contractions of words like mynameisjimandthisismydog.com.