In order to send a message to a potential donor, you will need to have requested their item or received an offer from that company. Once that happens, an option to send a "New Direct Message" (button) shows up near the top of the item's details page:
To get there from the dashboard, go to "Match Activity" (on menu on left), then find and click on the item listing about which you have a question. The button will show up near the top of the profile.