The head-bump is sort of like a forehead nuzzle between two men (I have only seen men do it. Although, admittedly, I am not the best source of information). Here is the rundown:
Step 1: See someone you know.
Step 2: Lean forward and press one side of your forehead to the opposite side of your buddy's forehead.
Step 3: Alternate to the other side of the forehead.
Step 4: Alternate yet once more.
Presumably, more often than not, one does the head-bump with someone who knows the drill. Otherwise, as experience has shown, you might end up standing there alarmed and confused. Ironically, for a relatively homophobic country, men here are very comfortable with the man-on-man contact. It is not unusual to see Congolese men holding hands while they walk or cross the street.
In addition to the head-bump, another standard greeting here is the triple cheek kiss. Most people follow this convention, but with such a diverse international community in Kinshasa, you never know for sure what you are going to get.