I have moved my project so that resides on an external thumb drive so that I can access the project from my laptop as well as my office machine.
I have installed flash builder on my laptop. What I can't do is simply open the project on the thumb drive from the new installation of flash builder on my laptop.
Must I import the project into my new flash builder installation?
Yes. Import the project but to keep it up to date across both version by setting the thumb drive as the directory for the project. That way, if you change the directory on your work computer to the thumb drive also, any changes you make between work and home computer will apply to both and you won't have to keep exporting .fxp's.