Welcome to the forum!
The tag that you put in your post will alert him. He’s been tagged before, so I’m sure he’s already working on the issue. And I hope our other friend DM’d him as well, so you don’t really need to DM him.
But if you’re wanting to DM another member, you can just click on their name above their post on a thread. The forum software will pop up a wind with a big “Message” button. Click on that and compose your message.
If you want to DM a member who’s not in your thread, just open up your profile (either a single- or double-click on the circle with your “U” initial, in the upper right corner of every page of this forum). Across the top of your profile, you’ll see some headings, including one that says “Messages.” Click on “Messages.” You’ll be taken to a page with your messages. There’ll be a menu along the left side of that page. At the bottom of that menu, you’ll find a “New Message” button. (It stands out on my screen because it’s the only blue item on my screen.) Click on “New Messages” and you’ll be able to compose your new message. Just start typing in (no @ sign there) the name of the user you want to message and the handy forum software will quickly provide you with a list of users whose name begins with those letters. So, you don’t have to do a bunch of typing. (And, of course, don’t forget to click on the “Message” button to send your message. )
Hope that helps.