If you have Outlook 2007, then that makes things pretty easy. Your Outlook program is 32-bit. Outlook 2007 is only offered with the 32-bit package.
If you don't have Outlook 2007, you need to first check which version of Outlook you have. You still most probably have the 32-bit version installed, even if you have a 64-bit Windows and/or machine. To be sure however, go through the below steps.
In Outlook, go to 'File'.
In the column on the left, if you see 'Help', then you have Outlook 2010.
To check if you have 32-bit or 64-bit package, look to the right of the version number.
In Outlook 2013 and beyond, when you go to 'File', if you see 'Office Account', click on this. To check if you have 32-bit or 64-bit package, click on 'About Outlook'.
The circle in the screenshot below indicates where you can find out if you have the 32-bit or 64-bit version.
Once you've determined whether your Outlook has the 32-bit or 64-bit package, you can download the appropriate Outlook plugin in 'Settings > Applications & Integrations'. 🚀