Did you install GFWL and RGSC? I need both of them to play in multiplayer.
You can only get the nitro in race missions, the icon on your map is the same flag as in San Andreas, they are unlocked after a mission with Mori I think. It doesn't give you any extra speed, just better acceleration.
The wheelie method works the same as cars, you are faster on a bumpy road. It won't really go over 180kmph if it's flat. On some roads you can get up to 230-240kmph.
If you pop your rear tyre and do a wheelie you'll be able to get over 230kmph easily on a flat road, but with great loss in acceleration and steering.