That's just the trade liaison having a meeting with whoever's in charge to set how much you're willing to pay for different things and how much they'll bring next year.
*To trade, you need a Trade Depot, which is a 5x5 building built using b, D. You put this down somewhere where the wagons can reach it, otherwise you only get pack animals. You seem to have done this step.
*When the merchants turn up, you need to send a dwarf to the depot to trade. There's a noble position called the broker, which allows you to send the same dwarf to trade with each caravan. Press n to go to the nobles screen, go down the list to the broker position, and select a dwarf. High social skills are good.
*Once you have your broker appointed, q over the depot and set it to 'trader needed at depot' and 'only broker may trade' (exact wording may be slightly different). The broker should come to the depot, although they're infamous for going on an eat-sleep-drink-On Break cycle during the month that traders are there. If that happens, change the settings to 'anyone may trade'.
*q over the depot, then g. This takes you to the screen where you select which items you want to bring to the depot for trading. There's a search function here, or you can use the list on the left to move between categories of items. Use the Enter key to select items.
*Wait for the dwarves to start bringing items to the depot, and for the traders to finish unloading. You can trade as soon as unloading is finished -- you don't need to wait until all the goods are in the depot.
*q over the depot again, and press t to start trading. The merchant's goods will be on the left side of the screen, and yours will be on the right. Press Enter to select what you want from them, and what you want to give them for it. If your broker has sufficient Appraiser skill you'll be able to see the prices, and if they have sufficient Judge of Intent skill you'll be able to see the trader's mood. They start neutral and their mood can be lowered to the point where they'll pack up and leave in a huff, so don't try and almost break even on your first trade of the fortress.
*Press t to iniate the exchange of goods. Hopefully the merchant's mood is raised by this, which gives you more breathing room for more exchanges with this particular caravan. The o button offers your goods without asking for anything in return. Don't use this one unless you want to give the caravan free stuff.