The program counter is part of the processor that contains the address of (the first byte of) the current machine instruction. In the register display (top window) you see that it is initialized to zero. This must be changed to 0x00400000 , the address of our first instruction. To do this, select (click on) Simulator; Set Value in the menu bar.
In the set value dialog, type PC in the top text box and 0x00400000 in the bottom text box. Click on OK and the PC (in the register display) should change.