Prepare the salad: drain the corn and black beans, and put them in a large bowl. Dice the bell peppers and add them to the same bowl.
Prepare the dressing ingredients: finely mince the shallot and either mince or press the garlic. Zest and juice the lime. A microplane works well for zesting the lime and for grating the garlic, if you have one available.
Mix the dressing: combine the olive oil, lime zest, lime and/or lemon juice, agave, minced shallot, minced/pressed garlic, cayenne pepper, and salt in a small bowl or in a food processor.
Combine everything except for the avocado, mixing well. If desired, chill for a few hours before serving.
Immediately before serving, dice or scoop out spoonfuls of the avocados and combine with the rest of the salad.