Just got the point where I have to go the Dead City, and it crossed my mind that I have no idea on how to find these extra missions. Can anyone give me a hint of where I can find them?

You need to actually search for them in the game. If you want to do it the easy way then just use this list: http://rage.wikia.com/wiki/Wasteland_sewer_missions_(DLC).

well I know I need to search for them, but I have no idea what to look for! I mean, is it random manhole covers in the dirt, or what?

The first one is located just above where you exit your Ark. Look for a guy next to a red tent underneth the overpass. He should be near one of the first two jumps.

You enter the sewers via these large raised manhole covers, they are quite big and once you see one you'll instantly know what it is.

Just outside the ark entrance (where you start the game) there is a guy stood nearby. Approach him and he'll tell you that the sewers are dangerous, and warns you not to go down there unarmed. He's standing right next to the sewer entrance. Press A to enter the sewers.

There's also a manhole cover in Wellspring, those are the only 2 I've seen so far.

god them sewers they will take you hours to clear ><

I'm okay with that!

Mostly every instance or settlement there is one nearby, look for the hatches.