Hi Rae,
You can only send emails to leads, contacts, and accounts. If this custom entity is related to one of these entities you can pull information from that entity dynamically and put it in the email, but the "to email address" will need to come from one of those three entities. The reason we do not send from other entities is all of the other records that come back with sending emails. The Email Events, Sent Emails, Unsubscribes, Subscription Preferences, etc. all need to be associated to the record you send to.
If you are not trying to actually send to this entity, but instead pull information from it, please let us know a little more about your situation and we can try to help you come up with a way to do that. You can respond here or if you do not want to post the scenario on the forum you can open up a support ticket here: http://support.clickdimensions.com.