The class quests are given by the Guildmaster inside the guild, and not out the front. The Guildmaster for the ARC Guild is a female elezen whose name eludes me at the moment - she should have a quest flag above her head (if the next available quest isn't currently selectable, it will be in red with a key icon on it). Chances are you've only done your level 1 class quest and hence have to do the quests that became available at level 5 and 10 as well (class quests become available every five levels up to level 30).