First, open the costing tab of the Preferences and input a hourly labour cost. Make sure you specify a preparation time in your recipe. The labour cost will be calculated as the preparation time times the hourly labour cost.
When adding a labour cost in the preferences, make sure however to decrease the default profit margin accordingly, as a profit margin of 72% takes already labour costs into account. If you don't decrease the margin, you will get a selling price to high.