I just became Thane in Markath. I can't seem to find my house carl. I assumed he'd show up at Vindrel Hall, but he hasn't. What am I doing wrong?

Either leave Vlindrel Hall and enter again, wait 24 hours in-game, or a combination of both. This usually resolves problems like these.

So he is supposed to bein Vinrel hall, then?

Yes he is supposed to be in Vinrel hall.

I have the same problem. I've been thane of Markarth for awhile now, and have been back to the house several times, and have not seen the housecarl anywhere. I'm not that worried with Janessa around, but it would be nice to have another mannequin to trick out with looted spoils from exotic locales, if nothing else.

I just want to let you guys know that the Markarth Housecarl ,Argis the Bulwark, that is suppose to be in your house protecting your stuffs is most likely out chasing girls around as I personally witnessed mine doing near RIFTEN! I almost fired (or made him as another test subject for my latest research at the college) his ass had anything gotten stolen from my house.

http://i1096.photobucket.com/albums/g322/Raidensama/Skyrim/TESV2012-03-2500-09-13-60.jpgAt the time I did not have any followers nor had one for sometime. I saw him chasing this skimpy looking girl down as if she was in heat.

haha...that is awesome. So, i guess i should look in Riften then? Maybe him and Iona is getting busy in Honeyside. Nice grab though, you cold busted him. LOL. But can you blame him? You know how hot the women are in the south!

Go speak to the Jarl again. I had this issue in both Markarth and Riften. I'd done everything needed to become Thane---done the favors for the citizens and bought the homes, but no housecarl. The problem was that I hadn't gone back to the Jarls to tell them I'd bought the properties. Once I did that they declared me Thane and gave me the housecarls. Hopefully that solves the problem for the OP.