I just did some testing and confirmed that the same procedure still works for me, then I re-read your steps and saw that your problem is different to mine (Sorry I was working off memory). You are getting the busy notice immediately after selecting a project to sync and cannot seem to progress past there. I too get the busy notice at that point but it goes away immediately after the first retry, which I press straight away. I'm guessing here, but your problem may be to do with the size of your database, or the programs you are selecting. Let me explain my server busy issue and it might make sense.
After selecting a project to sync and clicking the retry button I get the Field Selector window, and after entering details and clicking OK a new data source appears in Navisworks. I then have to "Rebuild Task Heirarchy" this data source to actually import activities. About 20 seconds after pressing the re-build button the server busy notice comes up and that is when I have to wait my 2-3 minutes. Now I did some more testing this morning and noticed that the "wait" time depends on the size of the project. I've been using this on a 1000 activity program but when I tried on a 99 activity program I still got the busy notice but it finished the re-build immediately when I pressed the retry button straight after it appeared. I then tested other projects
1. 99 Activities - 0 Wait time
2. 300 Activites - 20 seconds
3. 650 Activities - 1 minute
4. 1000 Activities - 3.5 minutes
5. 8000 activities - 2 Hours
So the relationship between server busy time and number of activities is not directly proportional, but increases exponentially with increasing file size. So my previous theory was wrong, and I think now it’s not a matter of the database being busy at all, it’s more likely that this is the default message when it’s waiting for the data transfer to complete.
It might be a long shot but it could be a similar exponential issue with your database that doesn’t let you get past your step 4. My database has a total of 114,000 activities in it and is 2.3GB. It’s also a local XE database. If you are trying to connect to a database on a WAN then that would definitely cause an issue.
One other thing to check: You need to have “superuser” privileges on P6 for the SDK transfer to work.
For the record I'm using:
P6 8.1 Professional on a local XE database
Navisworks Simulate 2013 SP2
Lenovo,W520 i7
16GB