I've gone through quite a bit of trial and error to solve this problem. Here is where I am right now.
I thought that I would try to install ACT 10 on the same pc that ACT 5 was running on. ( I did this when I tried the above linked article - I tried the procedure once by just copying the files and once installing ACT 10 with ACT 5 already installed. I used virtual machines) I could not get the inbox to come into ACT 10.
Next I tried to install ACT 9 on the pc that ACT 5 was installed on. This mostly worked right away, with no extra steps. When I was done installing ACT 9, all of my internet mail folders were there, but the inbox and sent items were not. Progress, but not perfect. I tried to do the actdiag, but it didn't work at all, couldn't even see the databases.
For shiggles, I installed ACT 10 over ACT 9. This was the first install that seemed to do an in-place upgrade. Everything that worked in ACT 9 worked in ACT 10 - still no inbox or sent items.
This really seems far more difficult than it should be.