How do you add a tip to a meal expense?

Depending on how you want to capture that expense you add that amount to the amount of the actual meal expense amount.  For example:

Meal $12 (Meal $10 + Tip $2) 

Tax $2 = $14.00 total expense 

Your entry for the expense would capture the meal + tip as one amount. If you want to see all the tip amounts separately you could enter a new line item for the tip amount.  

In Canada you can only deduct 50% of any meal/entertainment expense, so to illustrate per the example above (not breaking out the tip amount):

Meal/Incidentals $13 ($10+2+1)

Tax ($1) = $14.00 total expense 

This will capture your correct amount of claimable tax as well as the expense of the meal (which includes the tip and the non-claimable tax portion).