I'll assume you're on PC. Consumables will activate when you have a consumable equipped, you have satisfied the required conditions for their activation, and you press their associated key (on PC, this will be keys 4 - 6 by default). Only one consumable activates automatically: the automatic fire extinguisher, which activates when you're on fire.
The consumables that require crew/module damage are the Repair Kit, Medkit, and Multikit, which can't be used unless you take module damage, crew damage, or either or fire respectively. You'll know it's been activated because it's icon on the right will darken and have a timer superimposed upon it, and your module/crew damage will go away from the bottom of the screen.
There are also active consumables (Adrenaline, Engine Boost, etc.) that can be used without any prior condition, last for 15 seconds, and have a cooldown timer after being used.
You buy and equip consumables to individual tanks in the garage; its one of the selections in the row at the bottom, along with other helpful items like provisions, camo, and equipment.
Edited by Kiwi_Kebab, 30 April 2020 - 03:07 AM.