If you just want the sound, try dropping the realistic weapons pk4 in your base doom3 folder. In most cases, it will trickle down to sikkmod.
Otherwise, use this method:
1) Make a copy of the sikkmod folder in your C:Program Files/Doom3
2) Rename the copied folder to "realistic weapons" (lowercase is best)
3) Go in your new realistic weapons folder and find description.txt, open it in notepad, change "Sikkmod" to "Realistic Weapons"
4) find pakmodfiles.pk4, rename it to pakmodfiles.zip
5) rename weaponfixbykilomate.pk4 to weaponfixbykilomate.zip
6) drag and drop all files from weaponfixbykilomate.zip to the same folder in pakmodfiles.zip, if you are asked, overwrite any pre-existing version of the same file
7) rename pakmodfiles.zip to pakmodfiles.pk4