Share variables between verilog file and niosII application file(.c file)

Sure, I see you are really confused.

And so am I, since I can't understand what's you point.

I guess SharedVar is defined and assigned inside st_0 MM slave device, but you are trying to read its value from onchip_memory2 !!!

Then, that's the main problem; the correct base address to be used in IORD is 0x41040.

Anyway there are other major flaws and I think you need to learn the basics of Nios system before you can go further with this design.