To find the Client ID: Keep in mind that Campaign Monitor may change their interface at any time without notice so these exact instructions may not be up-to-date)
1) Sign in to your Campaign Monitor Account.
2) Click on the Clients link in the upper left hand corner.
3) Click on the Client you would like to use.
4) Click on Client Settings.
5) Click on the Edit button.
6) Click on the "Show client's API Info" link.
7) Copy the Client ID.
