Hmmm... these days, MPEG-2 is rarely used by video streaming services. H.264 (MPEG-4) yields comparable results and is far more data conservative.
In Compressor, in order to edit a preset, you have to duplicate it first (Cmd-Click the preset and select "Duplicate" from the contextual menu), which will in turn create a copy of that preset in the "Custom" settings folder. You can give this setting a new name if you wish in the Inspector window.
In the Inspector window, you'll notice that there is a group of six buttons near the top. You'll want to click the second button from the left, which switches to the Encoder tab. Here, there are three tabs near the bottom of the window. Click the "Quality" tab and you'll find the bitrate adjustments settings there. Under the "Mode" pulldown, you would select "1-pass CBR" and you would enter 6.0Mbps under "Maximum Bitrate".
There are number of other settings you can adjust in the Inspector window to further tweak the encoding (like Frame Controls, for example), but most what I explained above should at least get you started without getting too technical.
In any case, when you're done adjusting settings, you'll want to click the "Save" button on the bottom of the Inspector window to save the changes to the setting. From there, you can drag the setting to a job in the Batch window and submit it for encoding.
