NetSim provides software builds of 32 bit and 64 bit starting from NetSim v10.0.  

Linker errors will be displayed, whenever a user has a 64-bit build of NetSim installed in the PC and attempts to build a 32-bit dll from its source code or vice versa. 

If this is the case, you will have to change the solution platform from Win32 to x64 or vice versa as shown in the screenshot below, before building any project.