To solve this problem, please follow the steps below:
1. Download the attached dll (32-bit dll for 32-bit NetSim and 64-bit dll for 64-bit NetSim)
2. Rename the dll as libIEEE802.11.dll
3. Paste the dll in <NetSim's install directory>/bin folder (For example: C:\Program Files (x86)\NetSim Standard\bin). This will overwrite the existing libIEEE802.11.dll