The interface component we created to connect the SL811HS to the Avalon bus has an extra facility for enabling/diabling interrupts. The bit to allow interrupts to pass through lives in the 3rd register of our interface component (usb_base_address+8) at bit 2. Writing a 0x4 to this register will enable interrupts from the SL811HS to reach the Nios processor.
I am emailing you the most recent version of our document describing our USB reference project (a similar document should have come on the CD with your board). It contains a description of our SL811HS-to-Avalon interface.
Dennis Scott
Microtronix Datacom Ltd.