There are many ways to select a meal plan. Depending on the way you selected it, there may be a short delay in reviewing the application and making the plan active. The general rule is that it will work by the next business day. If you sign-up on a Friday, that means it will be active on Monday.
If you use the online meal plan sign-up, an email is sent to your UB email address within minutes. You must click on the link in that email to confirm that you requested it. Then the application will be reviewed and processed on the next business day.