Welcome Onlysimioli. If the plastic pin from the slider (first picture; middle part of plastic slider above spring sleeve) broke, your best bet will be to claim a warranty claim/fix through gerber's website. Also, check CL or the Bay for a "used and abused" one but with good handles to use as spare, as it is easier to replace handles than to try cobbling up a fix; there are no consumer replacement parts available from Gerber, and I can't think of someone who might have just a slider. Also, you'll need a rivet and have a way od peening it without breaking the "new slider".
Well, if it isn't broken, place spring on slider pin, push with needle nose pliers or small screwdriver, to compress spring and place over the handle side's pin. Check function.
Hoping that you'll be able to have it back at full service soon, and that you're safe.
Please let us know how it ends up, and be sure to post pictures; MP's are some of my favorite MT's!