Being named Hortator of House Hlaalu is pretty much a matter of twisting the right arms and greasing the right palms. It's nice dealing with a group that believes that everything has a price.
You'll need to get all of the House Councilors on board with the idea, though, so you've got a little traveling to do.
Crassius Curio (Plaza level of the Hlaalu Canton): grease his palm. You don't even have to take off your clothes. It will cost you 1000 gold if you are not Hlaalu, 500 gold if you are Hlaalu or 300 gold if you are Hlaalu and his disposition is 70 or higher.
Yngling Half-Troll (Yngling Manor, St. Olms Canton): grease his palm. 1000 gold if you are Hlaalu or 2000 gold if you're not. Or simply grease him. None of the other councilors seem to care and a couple of them will even suggest it. Taunt him unless you want to pay off a 1040 gold bounty, though.
Dram Bero (Haunted Manor, St. Olms Canton): talk to him. He's agreeable, but points out that getting the other councilors on board is going to require that you do something about Orvas Dren. If you killed his champion in the Arena and are running under the second patch, you're going to have serious problems with this.
So Orvas Dren is going to be the sticking point. Cheer up. You may have already solved the problem. If you have followed through on all of the Thieves Guild quests, you already have the material to blackmail him into submission. If you have followed through on all of the Morag Tong quests, you already have the material to blackmail him into submission. If you have followed through on all of the House Hlaalu quests, you have already killed him or blackmailed him into submission. If none of the above apply to you, you can either kill him, blackmail him into submission or convince him to name you Hortator.
Killing him: a tough fight, especially because his plantation villa is a bit cramped and there isn't a whole lot of room to maneuver. He's got one guard in the house that you should probably take out so you don't get attacked from behind. Aside from that, it's a straight slugfest. Dren is sporting full Orcish Armor and an Ebony Spear (65% skill) and has 250 HP. After killing him, and if you are running the release version or under the first patch, you can then go talk to Duke Dren in Ebonheart to be named Grandmaster of House Hlaalu (even if you belong to one of the other two Houses). If you've installed the second patch, this bug was fixed.
Blackmailing him: another tough fight, but with the Ienith brothers in the basement of the villa. Collect the papers and tell Dren that you have some papers that he should see.
Convince him: if you can get his disposition up to 70 or more, you can convince him to support you by telling him that you want to defeat Dagoth Ur and protect Morrowind from the Empire.
Anything else will either drop his disposition or provoke a fight.
Nevana Ules (Ules Manor, across from Suran): admits that she won't do anything with out Orvas Dren's okay. If you have already taken care of Dren, she agrees to naming you Hortator.
Velanda Omani (Omani Manor, across from Vivec): suggests that you talk to her advisor, Orvas Dren, about such important matters. If you have already taken care of Dren, she agrees to naming you Hortator.
Once all of the councilors are on board or out of the way, go back to Curio to collect the Belt of the Hortator.