Make sure to register the DecoderComp.dll in your system before running scarletbook on DST encoded SACDs. To register it, run the following at the command prompt (on Windows 7 or Vista, make sure you're running as administrator)

regsvr32 c:/path/to/DecoderComp.dll

on the command prompt, navigate to where scarletbook.exe resides and place the ISO file in the same directory. The following are few examples: 

scarletbook -h			- Shows the available options
scarletbook -iIMAGE.ISO		- Extracts the DSD stereo files to DSDIFF (note the lack of space after the '-i') 
scarletbook -c -iIMAGE.ISO	- Extracts the DST encoded stereo files to DSDIFF
scarletbook -c -m -iIMAGE.ISO	- Extracts the DST encoded multichannel files to DSDIFF

Alternatively use the batch files provided with the ISO files located in the same folder.  