for your tool library to load as default you have to edit the machine definition. start a new session of mastercam, load the Machine from machine type pull down menu
settings
machine definition
edit general machine parameters
tool/material libraries
set the library
save definition
be sure to do the file save first ( save icon ) before the green check
it should load up everytime after that. if you still have problems then your config file may be corrupted. delete it and start MC and it will rebuild the config file. at that point edit the MD
If you can ENVISION it I can make it