That depends. If you drive from your main job straight to the ball diamond--in other words, you're driving between jobs--you have deductible business miles. But you can't deduct the miles from the ball field to your home as this is nondeductible commuting mileage. If you go home from work first (or cover games on the weekend) and just drive from your house to the field and back, you'll only have nondeductible commuting. The diagram below from IRS Pub 463 shows when your transportation expenses are deductible.
It's unlikely the cost of your meals are a business expense. This is because you can't deduct meals unless you are away from home overnight (or long enough so that you must sleep or rest to properly perform your duties). On the plus side, if you go out of town to cover a tournament or playoff series you can deduct your travel costs, including lodging, mileage, parking fees, and tolls, plus 50% of the cost of your meals.
Remember to keep contemporaneous records and receipts of all of the business expenses associated with your umpiring duties. Not only will this save you time when you get ready to file your tax return, but you'll need this information if the IRS ever asks you to substantiate the expenses.
For more information, including how to keep records and prove expenses, please see IRS Pub 463: Travel, Entertainment, Gift, and Car Expenses
