OpenRocket has evolved a great deal in the 10 years since OR 12.03 was first available.
As to your question, this evolution includes the incorporation of Dave Cook's parts library and all of the ThrustCurve motor files, both of which are regularly updated. Those who use certified motors should see a significant decrease in need, most will have no reason to add motor files to OpenRocket, save those few on the cutting edge who may need files for just released motors during the period of time between certification and inclusion in the ThrustCurve database. And, being able to "uncheck" manufacturers and the use of other filters takes away most user concerns regarding seeing undesired motors. For those using experimental motors, motor files for those may placed in a single common directory and referenced (added) as before on the Preferences -> General tab.
Insofar as editing OpenRocket's internal motor database is concerned, this is not recommended unless you are experienced in Java and OpenRocket's data file structures. Quite frankly, if you are running the current Beta version of OpenRocket, I'd be interested to hear why you are trying to modify the internal motor database, what is it that you are trying to achieve?