I'm using MBLab 7.01, so that's what these instructions are targeted for.
To bring up the simulator run controls, go to Debugger menu -> Select Tool -> MBLAB SIM.
To view specific aspects of the simulated PIC, go to the View menu, and from there you can select program memory, file registers, special function registers (which is where you would get your simulated output pins), and such.
For input, go to Debugger -> Stimulus Controller -> New Scenario. Click in the first column under the Pin heading. Select which pin you desire from the dropdown menu. For the Action column, there are a few options. Set High, Set Low, Toggle, Pulse High, Pulse Low. Toggle is probably what you want, so that you can use a single Fire button ' > ' there in the first column of the stim controller box to turn a pin on and off.