In order to add a trailing stop, right-click on the price axis, or DOM Bid/Ask Columns and select from the dropdown, "Trade"  From the following dropdown menu, select the type of trailing order to be placed. 

You can also set a key to the trailing order.  To do this, click on Configure > Settings > Orders > Submission. From this tab select which key configuration you wish to link to trail order. 

Press "Apply" then "OK".