If you are using an Intel Mac, you'll need to run Strata 3D CX 5.1.1, 5.5.x or 5.6 in Rosetta mode. Select the Strata 3D CX application icon in the Finder then use the Get Info command. In the Info dialog expand the General section and turn on the checkbox "Open with Rosetta".
This is not necessary if you are on a PowerPC computer.
You can open the file in one of those versions to convert it, save it out and then open it in CX 6.x.
There is no guarantee that everything will transfer properly, but most of the model should make it through this process OK.