Current version: 2.9.0 - Mar 03, 2014 - ChangelogChoose the version of the Onetastic add-in based on your OneNote installation, not Windows. If you are using 32-bit version of OneNote choose 32-bit, otherwise choose 64-bit, regardless of your Windows bitness.
If you don't know what version of OneNote you are using you can find it out from within OneNote.
For OneNote 2010:
Click on File > Help in OneNote and check the version on the right side where it says something similar to:
For OneNote 2013:
Click on File > Account in OneNote and then click on About OneNote on the right side and check the first line in the dialog that comes up which should say whether it is 32 bit or 64 bit at the end.