In NX2406 the Tool machining data tab is missing when editing machining data library.
If we compare the menu for NX2306 and NX2406 we see the tab is missing in NX2406
The Tab "Tool machining data" has been removed and this is because in the upcoming releases of NX the focus will move to Cloud Connect Tool Manager for tool data attribution.
More info on CCTM here:
https://docs.sw.siemens.com/en-US/doc/209349590/PL20231101866122454.mfggeneral/xid2273083
Once in Cloud Connect Tool Manager a tool can be selected and individually assign machining data parameters to the tool.
And to keep old data from previous version, you can copy the previous .dat file (not .def and .tcl files that need the 2406 version).