If this is simply because you want your own domain name for your blog on wordpress, you can do that directly through wordpress. If not, do the following:
First of all, you should be able to point your domain name to iDisk. With some help from bluemukaki and others, I managed to do this with my own MobileMe account. I’m assuming you have already bought the domain name. I was going to describe the steps to you, but found this that pretty much explains it how I would have.
However, the CNAME part can be a bit tricky. You need to log into your account with your domain name provider and then go to the domain name’s settings, where you should be able to access a section called something like ‘DNS control’ or something similar. Here, there will be a list of CNAMES for your domain name. You might have ones such as wap, e, ftp, email etc. If you chuck any of these extensions in front of your domain name, they will point you to whatever is selected next to each of them (e.g. if you type e.yourdomainname.com, it will take you to whatever is listed next to the ‘e’ CNAME record).
This might all seem complicated, but all you need to do is this – or something similar, it’s slightly different from provider to provider. Click on ‘add new CNAME’ or whatever, and put the values listed in (www and web.me.com). This is basically what the document I linked in the first paragraph gets you to do. Once that’s done, you chuck the domain name into MobileMe and it should work. If your domain name provider allows it, you might want to turn on masking (this will simply hide the destination url and keep displaying www.yourdomainname.com in the address bar).
As for the wordpress site, again. I don’t know that you can ‘port’ it to mobileme, and setting the domain to redirect straight to the username.wordpress.com url would be much simpler. Hope that helps…