Before adding labor to a Repair Order, make sure to set Labor Rates in Settings 

https://shopboss.happyfox.com/staff/kb/articles/15-setting-labor-rates

Once a Repair Order is created, and Vehicle Issues have been added, https://shopboss.happyfox.com/kb/article/180-create-an-ro/

click the Labor button for the corresponding Vehicle Issue

A window will appear

Fill out all of the information. 

Fill out or click the drop downs labeled 1 through 6 to add Labor info, and click Save Changes.