how can I use the rootkit and are there any squads with room

Don't you just buy the weapon and then equip it? It should show in your cargo. May have to unequip something else.

u need to buy MLK-Inz Mining Laser Blueprint (10k) and MLKz rootkit (1k). Unmount your Newbie Laser from Equipment to Cargo and u can manufacture one MLK-Inz Mining Laser with 20 Mining-Power and 10 Damage.