With the newest download offering there are two different programs. One is command line based and the other has a graphical user interface. I don't have any OMA files but the GUI version looks pretty straight forward and shockingly close to what SpeshulEd predicted: Pick an OMA input file and pick your output file, which looks like .wav only so far, and push the "Go Render" button. There's also a batch mode option as well as some options for block size, offset, and smoothing settings. You also need the latest version of Sonic Stage.
Where are you having problems?