From another workstation that has Online Express installed:
  1. Click on Online Express in the navigation menu
  2. From the main Online Express page (Donations, Email Marketing, etc.), click on Help in the upper right
  3. Click on the About link in the lower right
  4. The About Online Express details, including the ID, will appear in a new box. Make note of the Online Express ID and click OK to close.
OR

Contact Support and reference this article.