I have Motherboard: X8SIL-F, Chassis: 813MTQ-520CB, Heatsink: SNK-P0046P;
the included air shroud does not seem to fit. Am I doing something wrong or is there another air shroud that is needed?
The default air shroud (MCP-310-00029-01) is for Intel DP motherboard. Since you are using UP motherboard (X8SIL-F), there is no need for another air shroud.