For enabling the LVD interrupts you have to set the LVD level either from the device editor or from VLT_CR register. For enabling the interrupt you have to set the LSB of INT_MSK0 register. This bit is responsible for enabling the interrupts for LVD. For revising the LVD ISR, you have to place a jump or a call instruction to the self defined ISR placed in assembly or C language. For placing the LJMP or CALL instruction you should modify the boot.tpl. Modifying the contents in boot.tpl will preserve the contents even when the project code is regenerated.