javihyuga: You must have it equipped, in your hand, being dressed as the driver (unless you want to be shot at) and you must face the car's door nearest to the restaurant (driver seat). If you satisfy these requeriments, you can select the "place bomb" option and the bomb shall be planted after a short animation.
Watch out, because if the guard sees you planting it, no matter if you are disguised, you will become cheese, because of the holes they will kindly make.

sneakcity: I also recall an alternative, or I'm talking about a different level. An underground tunnel leads to a metal ladder, which ends at street level in a manhole cover. I recall the car being parked directly over it, and I could plant a bomb from underneath. Something to consider if the driver is giving you trouble, or you want an approach with fewer witnesses. But I also recall it took some speed to get there in time, so you might wind up running in the underground tunnels to make it work.

sneakcity: I haven't played the expansions yet, but it could be from Hitman 2. A level where 47 targets a military figure in the open, in a park. I recall swarms of enemies with guns, such that as soon as you become a threat, you have a big problem! Might be a level where you board a boat to escape, so maybe Hitman 2.

Drainman100 - Was this level original "Hitman: Codename 47" or something else?