I assume your installation folder was not in root directory. You created the folder called "dotnetnuke", dumped all DNN files, and started the installation. Am I correct? You can do two ways:

1. Start from scratch. Delete all DNN files, dump all dnn files in the "root folder", and start installing the DNN.

2. or, you can place simple javascript, ASP file, or even php file on the root folder to forward website visitors to the http://www.mypage.com/dotnetnuke.  This might be better solution for you because, in the future, you can develop your new website under http://www.mypage.com/dotnetnuke2. When you finish developing it, all you have to do is to redirect people to the new website address(folder), which is dotnetnuke2.

If you installed the DNN in root folder and still not getting to http://www.mypage.com , you should add "alias" in host setting.