Go to "Options", then "Keyboard", then "Advanced". Check the "Developers Console" option. Press ~ during game play to display the console window. Then, enter one of the following codes to activate the corresponding cheat function:

Free bomb, as terrorist: code buy c4
Buy a frag grenade and stand next to the box that you want to get on. Throw the grenade opposite the box from you onto the floor. Jump, press against the box, and you will be boosted very high. If timed correctly you can get yourself on the box at a cost of 40 HP.