How do you use the half life jetpack

Jetpack? There is no jetpack. If you're referring to the Gluon Gun however, use this weapon with discretion. It tears up your enemies into bits or nothing within a matter of seconds. The maximum number of the ammunition is quite limited (100) and ammo is scarce. It'll come in handy for the final boss.

You mean the 1 in HL:DeathMatch??
it isnt a jetpack...