Hello,
I don't know if you have an answer to this already but I will post it here anyway in case anyone needs it.
It's just my experience and maybe I'm not doing it "by the book" but it worked for me until now.
There are 2 answers that depend on what P6 software you have:
A. Web version (or through Citrix)
1. Go into Navisworks -> Timeliner -> Data Sources(tab) -> Add(drop down) -> From the drop down select the P6 version you have (V8.3 works with newer version as well) -> Input Primavera credentials, Server address, Server Port (these can be given by your IT dept.) -> Select the project.
2. Right click on the source listed and from the drop-down select "Rebuild task hierarchy" and then you will find the P6 activities are added to Timeliner.
B. Standalone (or Web Services if you can't find the server details*)
*This implies that you have permission to export from Primavera P6.
**You might also need Microsoft Project to solve some issues that appear during export which will be detailed below.
1. Export the project in .MPP Format (You can also export as .MPX or other but the advantage of MPP is that it retains the WBS structure and you don't need to manually indent each activity in Timeliner)
2. Go into Navisworks -> Timeliner -> Data Sources(tab) -> Add(drop down) -> From the drop down select Microsoft Project 2007-2013 -> Browse for the exported file and open it.
3. Right click on the source listed and from the drop-down select "Rebuild task hierarchy" and then you will find the P6 activities are added to Timeliner.
Just be careful because problems can appear in the export process from P6 to MS Project so they need to be checked. (that's why it is recommended you also have MS Project installed). In my case the baseline isn't exported correctly so I have to manually adjust it in Primavera (I do this by copying and pasting two Excel columns not each activity by hand but still...). One other disadvantage I have is that I don't have the rights to create export profiles in P6 so I can't adjust the parameters from P6.