how do I enter actual vehicle expenses in lieu of standard mileage rate?

[KrisD15]

You have the option of choosing that option as you enter the expenses in the Business Vehicle Expense section. 

Business Tab

Business Income and Expenses

Edit the job on list

Scroll down to Business Expenses

Scroll down to Business Vehicle Expense    SRART or UPDATE

[uncpilot]

More specifically, how do I enter actual vehicle expenses in lieu of standard mileage for armed forces reservist travel expenses (>100 miles from home)?

[AnnetteB6]

Travel expenses for an armed forces reservist are entered on Form 2106.

You can find that form by going to the Deductions and Credits > Employment Expenses > Job-related Expenses section of your return.

The following TurboTax help article will give you more information and specific guidance if needed:

Where do I enter job-related employee expenses? (Form 2106)
 

@uncpilot