Firstly, have a read through this thread: viewtopic.php?t=66
With trueSpace 7.6 you can still create COB and SCN files but it it a little hidden. Going from memory, you need to select an option in the top right hand corner of the screen that will change your view to the trueSpace 6.x interface. From this workspace you can choose File>Save and then you'll have the option to save COB files.
I don't have trueSpace installed on my PC anymore so I can't give you exact instructions so I hope you're able to find the option that I'm describing above.
When you do get it working, could you post some screen shots in this thread of how to make this change for others to see?