How to call tightly couple module during C code for NIOS II

Think of the verilog you're writing as a bunch of wires. You don't need to give your main.v module access to the PIO port wires. There should only be 1 driver on a line or the result is undefined. If you want to toggle your LEDs or use buttons on an eval board, you can create an assign statement or you could control the LEDs directly through a separate PIO in the Nios. What you're trying to do is drive 'result_data' from the Nios AND from an I/O pin on your board somewhere. Similarly, 'int_dataout_0/1' is a wire and can't be defined as both a wire and IO port. In your port declaration of main.v, you need to come up with a different name (i.e. output [7:0] LED_RED; but double check my syntax). Use 'assign LED_RED <= int_dataout_0' if that's what you're trying to do.

For now, remove the port declarations and just program the Nios to printf the result to your debug console. That's the easiest way to see if the numbers are correct.

Also, for a simple addition you don't need synchronous logic - get rid of the clock unless you want it pipelined. In that case, you should really be using a different clock phase (with a PLL) than your SOPC system to ensure data is valid.

You can ignore the testbench warnings until you start simulating your Nios processor with Modelsim or another HDL simulator.