If you have not already, please join the DiFX-Users mailing list - to do this, go to https://listmgr.nrao.edu/mailman/listinfo/difx-users and sign up.
You might find it enlightening to browse the SVN source tree.
Read the Reference Manual which has a lot of detail about the NRAO installation of DiFX and a lot of information about the helper programs for configuring the correlator and managing the output.
The (now-defunct) forum on the old DiFX-users Google Groups page (https://groups.google.com/forum/#!forum/difx-users) might also have useful information, so feel free to swing past there and search old threads for answers to questions you might have.
difxmessage logging information
vdifio vdif access libraries
espresso (scripts for managing file based correlation) documentation
difxdb (database extension of difx)
Subversion tips and tricks
Complex-sampled signals and vex files - an explanation of how to configure DiFX input when using complex-sampled data.
DiFX2.0 explained - the differences between DiFX1.5 and DiFX2.0
The DiFX memory size/datarate calculator (now available as a C program in the difxio library)