If you've still got the wort boiling, add your hops and boil for however long your recipe tells you to boil the bittering hops. Probably 60 mins. Add water to keep your volume up, since you've really boiled for longer than 60 minutes and about 10% will evaporate each hour.
If you're not boiling anymore, check how much headspace you have in your fermenter. Go to your LHBS and find the highest alpha acid hops you can and buy an ounce or so. It's optional, but getting some 5.2 buffer will help too. Add the appropriate amount of 5.2 buffer and boil the hops in an amount of water a little less than the headspace volume of your fermenter. Chill the hop tea like wort and add it to the fermenter. It should be good for 10-20 IBU's. Probably enough for a Corona clone (I'm assuming that's what a "mexican cerveza" lager kit is.
This trick helped me add a little punch to an anemic IPA kit and a IIPA where I screwed up hop additions.
"I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth."
Pondering: Roggenwein, Irish Red, Another Single-Hop IPA, Roggenbier w/ local wild yeast
Primary: Hefeweizen, Nugget Single-Hop IPA, Chamomile Wheat
Secondary: Saffron Metheglin
Bottled: Belgian Strong Dark, Saffron Wheat, UK Barleywine, Tripel, IIPA