The default setting in the simExchange is "basic trading." This setting gives you an orderbook that tells you how many shares and at what prices you can buy or sell right now. All you have to do is enter the quantity of shares and your order is guaranteed to be executed, but at market prices (no particular price is guaranteed).
The simExchange also offers limit orders in the "advanced trading" mode. You can switch your account to this mode from the Settings page. This mode will also give you Level II Quotes for each game stock.
In the advanced trading mode, you have two options for entering orders. You can either (1) type the price you want to pay per share and the quantity of shares you want to buy or sell in the order form OR (2) you can simply click on one of the existing orders in the order book.
The orders on the left side are orders coming from buyers (Bids), so these would be the orders to choose if you want to sell. The orders on the right are orders coming from sellers (Asks), so these would be the orders to choose if you want to buy. Then simply press either the "Buy" or "Sell" button to submit your order. You should then receive an update in the System Messages window about your order.
"Filled" means another order was found to match your order--for example, if you want to buy 1,000 shares at 45, there must be an order from someone looking to sell 1,000 shares at 45. If your order was unfilled or just partially filled, a link will be available to take you to the Order Status page for you to track your order. This page is also accessible under the Account Tab.