How do you plant c4 in modern warfare 2?

Answer 1

You can throw it using the "Throw" button on your keyboard, and detonate it using the "Detonate" button. These button configurations can be checked in the settings menu.

Answer 2

In multiplayer select C4 as your equipment then just press RB to throw it,

Then press RB again to activate it.

Answer 3

Double tap X, or when you have the C4 remote out, press the shoot button.