There can be several different ways to compile the fortran code. I used this
method and it has so far worked for me.

STEPS:
0. Download cygwin (www.cygwin.com); while installing, enable these two packages: 
gmp, mpfr (not selected in default cygwin install)
1. Compile the .f90 files with the flag -x f95-cpp-input
(gfortran -c -x f95-cpp-input *.f90)
2. First compile the ddpression.f90 and ddcommon.f90 files to generate the .mod files
3. In subsequent steps, you sometimes have to remove the flag -x f95-cpp-input to compile
(http://gcc.gnu.org/ml/fortran/2005-06/msg00207.html)
4. If not all files compile, do it individually for each file with and without the '-x f95-cpp-input' flag
5. Once everything is compiled, remove DDfield.o, DDpol.o, and CALLTARGET.o from this folder
(otherwise an error related to '_main' will show up
6. Now compile with gfortran *.o -o DDSCAT.exe (an executable file will be created)
7. Copy this .exe file and the .tab and .par files from /doc folder to a different folder
where you want to run your simulations
8. Once on clicking on .exe file, if your program gives an error regarding cygwin1.dll,
copy cygwin1.dll to C:\Windows\System32)
9. Now you should be ready to simulate as you wish.