There are two versions of Mimecast for Outlook available to download for Outlook clients. The bit version to use depends on your Outlook install, not your Windows install. If your Outlook install is a 32-bit version you will need the 32-bit version of Mimecast for Outlook.
Mimecast for OutlookTM v6.2
9th February 2016 | Build 6.2.1494.15300
Mimecast for Outlook v6.1 R2 should only be downloaded by customers still using Microsoft Outlook 2007. All other customers are advised to download the latest GA version above. Microsoft Outlook 2007 is no longer a supported environment from v6.2 onwards and will not install.
Mimecast for OutlookTM v6.1 R2
27th October 2015 | Build 6.1.1396.14780