Native Access 3 is the latest version of the software used to activate the Virtual Drumline library (and any other Native Instruments based software). Since VDL 2.5 is an older "legacy" product, activating with Native Access v3 may sometimes fail. This will depend on a number of factors related to your system (OS, processor type, etc.).
Other times, upon activating your VDL library, you may experience an error message stating "That path is invalid."
Native Instruments provides a "how to fix this" article, that may provide some ideas, but depending on your system, this may not ultimately solve the problem.
Here is a quick video tour of Native Access 2 (which also applies to version 3), and how activating in Native Access 1 can be a good workaround if v2 or v3 isn't cooperating.
We will continue to monitor how Native Access evolves, and in particular how it handles older libraries like VDL 2.5. Fortunately, once VDL is activated, it continues to function very well in the most recent versions of Kontakt.