Hello,
It seems that you have already installed DNN in /Dotnetnuke of your web site.
Now you have following options -
1] make the /Dotnetnuke as the web root of your web site (change Physical Path of web site in IIS) so that it will pick the pages directly from /Dotnetnuke and the site will show its content only by accessing domain name, no need to use domainname.com/Dotnetnuke
2] suppose your domain name is abc.com hence your DNN is at http://abc.com/Dotnetnuke
Following is the way to access the content of http://abc.com/Dotnetnuke at http://abc.com/ no need to add /Dotnetnuke at the end of URL -
Create a index page say index.html in web root of abc.com , set index.html as the Default Document and add following code in it -
<frameset rows="100%">
<frame src="http://www.abc.com/Dotnetnuke">
</frameset>
<noframes>
<body>Please follow <a href="http://www.abce.com/">link</a>!</body>
</noframes>
what it will does is -
whenever you browse www.abc.com it will show only www.abc.com in address bar but will fetch/show the content of abc.com/Dotnetnuke
3] DNN have a feature called as DNN portal/alias, login your DNN as admin and you can add as many as alias into it as per your requirement.
Hope this helps you.
Good Luck :)