Once tModLoader opens, you will notice the interface looks similar to vanilla Terraria but with extra buttons.
Warning: If Steam shows version 1.4.5 or 1.4.6 preview, you are on the wrong branch. Switch back to 1.4.4-legacy. download tmodloader 1449 updated
So you managed to download tModLoader 1449 updated, but it crashes on launch. Here is the fix. Once tModLoader opens, you will notice the interface