The best practice (learning by doing) is to use the example provided by uVision2.
You can find in \keil\c51\isd51\examples\generic_8052
But you must be 100% sure you know how to use the debugger, performing single-step through code, set brekpoints and run your application.
It is impossible to use the UART interface that is used for ISD51 communication for another user serial interrupt service routine.
ISD51 requires exclusive control over the UART since it is implemented via a serial interrupt routine.
However, if don't like I/O polling feature and you have a 8051 micro with two serial ports and the Serial Port 0 is used by default in your application and don't want to change that, you're still able to use ISD51 on Serial Port 1 for debugging purpose by simple changing the UART interface used by ISD51 inside Isd51.h file.
But you still won't be able to use software breakpoints and single_stepping in Serial Port 0 interrupt service routines.