You can check your installation either by looking into Help->Show Status Report, or in the startup log, that the plugin was loaded. Now open the Preferences dialog, and select the Toolbar Settings on the left hand side. There you should see a new Go driving action available in JOSM actions. Add the button to the toolbar.
Alternatively use the Tools menu.
The Ultimate WMS Turbo Challenge II (WMS Racer) is a simple car racing game taken straight from the early 1990s and whose map is replaced with a JOSM's WMS layer, such as Yahoo! Aerial Imagery. That being so, the plugin depends on WMSPlugin being installed. The game runs inside JOSM and it can only be started when at least one WMS layer is present (in addition to any data or GPX layers) and the "Go driving" (checkered flags) button is not grayed out. With multiple WMS layers, make active the one you want to ride on.
The main usage for the plugin is generating GPX traces as the car is equipped with an (early 1990s) GPS receiver and records a trace whenever you're driving. This is especially useful for the mapper who doesn't own a car or a GPS of their own. The GPS trace will appear as a new GPX Layer in JOSM after you're done racing, allowing you to see JOSM's features for working with traces. Note that you should not perform an OpenStreetMap GPX Upload with these "artificial" traces.
The plugin is currently in early development and is particularly missing a handbrake and skid marks on the ground.
Note that the WMS source you choose should be detailed enough for roads to be visible, as the car is not made for off-road driving.
The car is steered using the arrow keys, even though joystick would be much better.
Hit Escape to exit the game.
S toggles sound effects, a.k.a. SFX.
C toggles the cacti, a.k.a. GFX.
There's one cheat key.
If new tiles are not being downloaded as you drive away from where you started, it may mean that your WMS layer has a too high resolution and thinks that you are zoomed out too much to attempt downloading all the visible tiles. The tiles will be rather small, too. In this case exit the game, zoom out so that you have a couple hundreds meters in view, then right-click the WMS layer in the layer chooser and select "Change resolution".
Any other problems please report (e.g. on IRC).