If you do not see an 'Add Expense' button in the Expenses section, this means that your Planning Method setting is currently set to "Goal Based". The Goal Based planning method does not require income or expenses, instead looking only to goals, savings, and account information.
You can find and adjust your Planning Method setting within the Gear Icon > Settings > Methodology tab of each client plan. To learn more about the impact of each planning method option, click here.