Hi, I believe I know what you need to do in order to achieve this.
First of all you will need to login to you Domain nam provider in this case yours is yahoo.
once logined in you will need to get rid on the redirect and then edit the DNS settings on that domain name. You need to change the "CNAME" to "web.me.com" and then save these settings. This can take anywhere up to a few hours for this change to take place.
You then need to login in to your mobile me account, got to settings, (you will be aksed your password again) and then click on the link labeled "Personal Domain" Seventh one down.
You then need to add you domain name type it in then confirm it and your done, then go to iweb and republish the site and I believe that is all.
P.S. When buying a domain name i would recommend having a good look because $9 is a fair bit if you going to be using mobile me in stead of yahoo for everything. You can find good priced domains at just over $1 a year with just the DNS option at:
https://order.1and1.co.uk:443/xml/order/Instant;jsessionid=D31EFC8C1F38944893186FDAB96547B6.TCpfix150b?__frame=_top&__lf=Static
Anyway this is what I did my self about a year ago now how this helps, Wez: http://www.orleafy.co.uk