Found the solution on another forum:

http://social.msdn.microsoft.com/Forums/sqlserver/en-US/4691a3df-5ad1-480a-8d27-2bda7afd7078/sql-2014-upgrade-advisor?forum=sqlsetupandupgrade

Basically, you have to have the right version of SqlDom.msi installed. I believe the upgrade advisor relies on the 32 bit version.

Please on the following link:

http://www.microsoft.com/en-us/download/details.aspx?id=42295

Click on Download, then scroll down the bar with your mouse, and choose:

ENU\x86\SqlDom.msi (32-bit)

ENU\x64\SqlDom.msi (64-bit)