Check the manual to find out how to get it to give you a polar to rectangular conversion and a rectangular to polar conversion. Most scientific calculators manufactured today will do this. When you want to add two complex numbers, what you have to do is make sure they are both in the rectangular form. That way, you can add the real parts and the imaginary parts separately. The sum of the two complex numbers is basically the sum of the reals and the sum of the imaginaries.
Subtraction is the same - subtract the reals and subtract the imaginaries.
As you can see, this would be rather difficult to do if we had not expressed both of our complex numbers in rectangular form. In electronics we find ourselves using this technique a fair amount to solve AC circuits with both resistors and reactive elements such as capacitors and inductors. In writing our loop equations, we add resistors and reactive elements separately because of the way they work in the circuits. When you want to multiply complex numbers, they need to be in the same form - either both in rectangular form or both in polar form. If they are in rectangular form, the answer will be in rectangular form.