If you miss a dose, according to the consumer info:

Take the missed dose as soon as you remember, but no more than 1 hour after eating a meal. If it has been more than an hour since your last meal, skip the missed dose and take the medicine at your next regularly scheduled time. Do not take extra medicine to make up the missed dose.

If you miss a meal, or if you have a meal without fat, you can skip your dose of alli for that meal also.