You have to set the INTCON bits GIE and T0IE, can't recall of the top of my head but you may also have to set PIE.
Don't forget to clear the T0IF flag before you exit the interrupt routine. GIE will be set automatically with a RETFIE instruction, not sure how C handles it.
Also, TIMER0 is configured as a counter: T0_SOURCE_EXT choose an external clock as trigger. If you're using TMR0 as a timer, use T0_SOURCE_INT instead.
TMR0 interrupts are enabled by TIMER_INT_ON (the GIE bit must be set, as suggested).
Personally I don't use that function to configure timers, but it's described in the C18 C Compiler Libraries guide, not in the manual of the compiler.