Siemens NX 11 is an older version of the Siemens Digital Industries Software suite, used for advanced product design, engineering, and manufacturing. Since NX 11 has reached end of life, you cannot download it directly from Siemens' public website unless you have an active maintenance contract.
I notice you're asking about "nx 11 download" — but this could refer to several different software products (e.g., Siemens NX 11 for CAD/CAM/CAE, or something else). nx 11 download