The Power Plant Market window shows all currently available power plants in sorted order. The top row contains the 4 available plants and the 4 shaded plants in the second row are the future market of power plants. The panel on the lower left corner of the Power Plant Market is blank (gray), indicating that a plant has not yet been chosen for auction/purchase. You select a power plant by clicking once on it. Once you have selected a plant is shown in the bottom left panel.
In the picture above, there are also panels in the bottom row that look like stacks of yellow boxes. The left most rectangle is blank because in this picture no plant has been bid on by any player yet. The name "divinem" appears over the portion of the window having the bidding controls. There's an round white/green button that says "OK". This button is pressed when you enter a bid on the selcted plant. The wide green button labeled "PASS" is pushed if you choose not to bid on the currently selected plant. Note that in your very first game turn you must purchase a plant - game rules. The next panel to the right has the name of all competing players who have yet to decide on the selected plant. In our case "fastfingers" (me) is waiting to make bid or pass. The last panel on the right shows all players in the game that have yet to purchase a power plant this round. At this point in the game that's also still "fastfingers".
The first player, divinem, selected plant #04 and pressed "OK". I now see her name over the selected plant and my name over the controls panel. The current bid value is 1 higher than her opening bid. The current bid will be 5. If I press "OK" I agree to enter this bid and the we go back and forth until one of us presses "PASS". Note that the middle of the controls panel shows the current bid price and some +/- controls to adjust my bid before I choose "OK".
I moved the Power Plant Manager window down from the top of the screen. By default, until you drag it somewhere else, it opens up at that location. Moving it aside uncovers the green pane which helps you keep track of where you are in the phases of the game.
Power Grid is played in 3 Steps. BSW refers to these as "Stage". You can see we are at Step 1, Phase 2. The currently active player name is shown in the green window pane. The current action defined by this phase is "Auction Power Plants".