Take out your 9-inch springform pan. Pour the crumbs, sugar, and the margarine in the bottom. Mix with a fork until well blended and then press the crumb mixture along the bottom and 1 to 2 inches up the sides of the pan. (One technique to do this and stay cleaner is to use a piece of plastic wrap and keep your fingers on top of the wrap. That way you don't get brown crumbs all over your hands and in your fingernails, but for some that's half the fun.)
Bake the crust at 375 degrees for 6 minutes.
Take it out of the oven and make room in your freezer for it. You want to freeze it for 15 minutes. In order to not waste electric energy, you can let it cool on your counter first and then put it in the freezer if you have time. That way you also don't risk melting something in your freezer that happens to be nearby.
During the last 5 minutes that the crust is in the freezer you can make the mousse. You're just going to follow the directions on the box except you are going to put both boxes of mix in a medium bowl and make them both at the same time to get enough filling for your tart.
Put the mousse mix and the milk in the bowl and beat on high for 2 - 3 minutes until well mixed. The mix will thicken some and will become lighter in color. Pour the mixture into your tart crust and spread it.
Cool the whole thing in the refrigerator for at least 3 hours. You don't need to cover it but, if you want to, you can put a piece of plastic wrap over the top. Just remember it will stick some when you go to take it off, so your tart top will not be as smooth as it will if you leave it uncovered.
When you go to cut it, run a thin, sharp knife around the edges and then unlock and release the tart bottom from the side of the pan.
The tart is ready to eat. Cut with a very sharp knife and, if you don't want any of the tart to stick to the knife, spray the knife with oil or wet the knife first. You'll want to work fast so you don't make the cut-sides mushy.
Alternatively ... before you cut your tart you can decorate it.
You can put dollops of whipped cream around the top or on each slice.
You can sprinkle on strawberry halves or raspberries or even blackberries.
You can rim your tart with ground nuts. Pecans are good.
You can rim your tart with toasted coconut.
You can decorate with little edible beads or even M&M's.
The options are pretty endless. You are only limited by your imagination and favorite flavors.