Which one should I install? 64Bit 32Bit version?
The version of OLMixedFolders or any other Outlook AddIn have to match to the Outlook version. In order to find out which version you have do the following steps.
Which version of outlook do I use?
In Outlook 2010 click on file => help On the right side you will find your information.
In Outlook 2013 click on file => Office-Account => "Info".