The program counter is the part of the processor that contains the address of the current machine instruction. (Actually, it contains the address of the first of the four bytes that make up the current instruction.) In the register display (top window) you see that the PC starts out at zero. This should default to 0x00400000, the address of the first instruction. To change the PC value, select (click on) Simulator; click on Run Parameter; type the address in the top text box. Click on OK and the PC (in the register display) should change.
A user types "400000" into the value box, and clicks OK. The PC changes to 00061a80. What happened?