There are many ways you could represnet a deadline in P6 -as a constraint, or a baseline, or a user defined field for example. Which method you go with will depend on precisely what you want Primavera to do if an activity goes beyond it's deadline

Below assumes you do it via user-defined field:

1) Got to Enterprise menu > User Defined fields. Under "Activities" drop down menu, add a field called "deadline finish" with data type "Finish Date". Add another field called "deadline start" with data type "Start Date"

2) Display the Deadline columns, and assign deadlines to all relevant activities (if you only want to show a finish deadline rather than both start and finish, make start deadline = finish deadline)

3) Create a new filter called "has deadline" where "Deadline" "is not equal to" <BLANK>  

4) Right-click on the gant chart and select "Bars"

5) Add a new bar Name= "Deadline", Timescale = "User dates", User Start Date = "Deadline Start", User Finish Date = "Deadline Finish", Filter = "has deadline".

6) Use the shift down button to ensure it is below all other visible bars

7) On the "Bar Style" section, select the shape & colour you want the deadline to look like for left and rightmost sections (there is a down arrow much like the MSP version near the bottom of these options). Pattern should be filled bar. The middle section shape should be empty (the topmost option). Click apply.

You should get something like this:

deadlines.jpg