1) Extract SDRSharp.HackRF.dll to your SDRSharp install folder

2) Open SDRSharp.exe.Config and find the line "<frontendPlugins>"

3) After the line "<frontendPlugins>", but before "</frontendPlugins>", add this line:
   <add key="HackRF / USB" value="SDRSharp.HackRF.HackRFIO,SDRSharp.HackRF" />

4) Save and close SDRSharp.exe.Config

Notes:
1) You need to have compiled hackrf-tools for windows, or you can download my compiled copy from my website.
   http://the.midnightchannel.net/sdr/SDRSharp_Plugins/zefie/SDRSharp.HackRF.ZefieMod/hackrf-tools.zip

2) You will need to copy the 32-bit versions of libhackrf.dll and pthreadVC2.dll into the SDRSharp Program Folder.
   You can obtain these from either my copy of the hackrf-tools above, or, if you compiled it yourself, you
   should have both files in your development folder. libhackrf.dll may have been compiled as "hackrf.dll",
   you can simply rename it.

3) If you are using my copy of hackrf-tools above, you may need to install the
   Microsoft Visual C++ 2012 Redistributable, which you can get from Microsoft here:
   http://www.microsoft.com/en-us/download/details.aspx?id=30679