I've read the instructions at http://freearc.org/research/SREP.aspx and added SREP to my arc.ini but I can't find SREP in the gui? How can I compress my files with SREP? I've also downloaded the archive for SREP but I can't find any information where to put the files?
You should install latest (Feb 2012) freearc and replace arc.ini with the attached one. then use freearc GUI and select max. compression mode and enable checkboxes for srep/precomp compression. first line in the dialog will show you cmdlne options that you need. decompression may be performed by unarc.exe and require arc.ini and srep/precomp executables. it was about using precomp/srep inside arc archives
You can also use them "outside" i.e. archive files w/o compression (tar/7z -mx0/arc -m0 -dm0), then process this archive with precomp and/or srep and then compress output with 7z -mx/arc -mx