how to achieve the communication between niosII system and FPGA module

Look at the documentation of Altera's Avalon bus. These adapters (slave and master templates) are provided by Altera here: http://www.altera.com/support/examples/exm-list.jsp?cat=embedded

You may also search in this forum, there were vhdl versions of these adapters somewhere.

But I suggest you to skip on-chip RAM. I am sure, that your Nios II has its own memory, it would be smarter that your component writes data directly to that memory. Arbitration circuitry is crated by SOPC Builder, so you need just to provide write signal, address and data through your adapter to the Avalon bus.