1) In OpenIV, click File > Open Folder (Or push Shift + Alt + O). In the Select Folder window that appears, navigate to your /oldrpf/ folder you created with the dlc.rpf file you extracted to it and click Select Folder. In here you should see one Rage Package File, dlc.rpf. Right click this file and click Save Content/Export in the context menu that appears. Next, a folder selection window will appear. Navigate to your created unpack folder and click Select folder. This will extract the contents of dlc.rpf to this folder ready to be signed by ArchiveFix.exe
2) Open your ArchiveFix folder (the folder that contains ArchiveFix.exe). Then, in the unpack folder go through EVERY FOLDER (usually x64 + data) and every .RPF file you see, drag these file ONTO ArchiveFix.exe (oone by one). If you did this correctly, a command prompt window should appear telling you that the archive was fixed after each drag.
3) In OpenIV, click File > Open Folder (Or push Shift + Alt + O) and navigate to your newrpf folder. Making sure you have edit mode enabled (Click Edit Mode in the top right), In this folder right click in the whitespace on the RIGHT SIDE (where your dlc.rpf was in step 1) and click New, and RPF File, calling it dlc.rpf. This will create your new empty.RPF file. To put your files in this, double click your new rpf file and drag the files from your unpack folder into the right side of the window, where your .rpf file normally shows. DO NOT PUT DLC.RPF IN HERE, MAKE SURE THAT FILE IS NOT SELECTED WHEN YOU DRAG FILES INTO HERE! In the example of the rx7cwest addon, the only files in my dlc.rpf at the end of this guide were the Data folder, the x64 folder, a content.xml and a setup2.xml.
Hopefully you have better luck than us!