Recently did some computer "house cleaning", downloaded the newest MPLABX4.05, deleted both the v3.60 and v4.01. Afterwards, I also got "Unable to find mpasmwin". There is a PBP tool, on my computer it is at: C:\ProgramFiles(X86)\PBP3_1\MPASM-MPLAB_SETUP\PBP_MPASM_Connector. The wizard guides you to point to the file folder containing mpasmwin. In my case it is at: C:\ProgramFiles(X86)\Microchip\MPLABX\v4.05\mpasmx . After running that, everything started working again.

As for the K40, you MUST use PBP3.1 and you MUST use the MPLABX tools! Neither MPLAB 8.90 nor PBP3.0 have the necessary files for the K42.