Versions of Safari 6.1 and higher run the Silverlight extension in Safe mode, disallowing file system access.

Follow these steps in Safari to fix the issue:
1.   Go to Safari > Preferences > Security > Plug-in settings
2.   Press the Option key on your keyboard and click these three options: Ask, Off, On
      a.   Additional options are available but should be left as is!
3.   Un-check the "Run In Safe Mode" option to make it run the same as in earlier Safari versions.
4.   Click Trust.
5.   Click Done.
6.   Reload the browser page.

These steps should now allow you to log into your GIFTS Online site in Safari.

If these steps do not work, please refer to http://www.blackbaud.com/files/support/guides/microedge/giftsonline/GO_Config_Mac.pdf as this article will contain the latest details on how to set up your Mac device for use with GIFTS Online.