Background:
Here's what going on, if you don't care and just want to fix it, skip this bit.
The writers of HDSDR decided that the software should not transmit unless 'approved' SDR hardware is attached so they built special handshake into each ExtIO.DLL interface so TX functions would only work if ExtIO is loaded. Unfortunately, all suppport for HDSDR is stopped, including adding support for new hardware like the RS-HFIQ. For most control functions Omni-Rig works perfectly in place of ExtIO.DLL. Except transmit. So what we do is install the ExtIO_Si570.DLL which supports a Softrock transceiver. We do not use this DLL for any function other than enabling TX in HDSDR.
Some distributions of Windows, and at this point I haven't figured out exactly which ones, are missing a library that ExtIO_Si570.dll needs to load and do it's HDSDR handshake thing.
Fix #1:
If you can't select 'SoftRock Si570' in OPTIONS => Select Input then download the file at the bottom of this page (LIBUSB0.DLL) and put it in the directory with HDSDR and ExtIO_Si570.DLL which is usually C:\Program Files (x86)\HDSDR.
Fix #2:
If you get an error that mentions "side by side configuration" it means that your PC does not some of the DLL's needed to run EXTIO.dll. The needed files are installed when you run the setup program for CFGSR.
1. Download "CFGSRSETUP.EXE.ALLOW" below, to your desktop or some convenient location
2. Rename the file "CFGSRSETUP.EXE" (remove the .ALLOW)
3. Run the file