If your WordPress site is not hosted by Blackbaud
Please contact the appropriate software vendor or IT professional for assistance with this process or issue which is beyond Blackbaud's scope of support.

If your WordPress site is hosted by Blackbaud
Plugins must be installed by Blackbaud. All plugins have to be approved by a Blackbaud Advisory Board, due to PCI Compliance. At this time, we are not installing any new plugins to the WordPress sites. We will update this article if this process changes. 

Additional resources
Can we get access to install our own plugins or upload files via FTP to our wordpress site