Right now, due to a limitation in how OFP: Resistance loads and manages addons, you should place all unofficial addons in the old OFP version's \Operation Flashpoint\Addons subdirectory. Note that this may prevent you from playing with the older OFP version, if any of the addons are for OFP: Resistance and above and are not downward compatible.
Now, some of you folks will be startled by the fact that you do not have an old \Operation Flashpoint\Addons subdirectory. Don't be frightened! This will occur if you installed OFP version 1.00 and then immediately installed OFP: Resistance. This occurs because the \Operation Flashpoint\Addons subdirectory was only introduced in OFP version 1.20. However, if you skip installing any old OFP upgrade patches and go straight from OFP version 1.00 to version 1.75 (OFP: Resistance), this folder will never be created by OFP's install or update procedures.
So, where can such people install their unofficial addons? Try manually creating the missing \Operation Flashpoint\AddOns subdirectory and test whether OFP will detect addons you place there. In any case, a better alternative would be to use the -mod command statement parameter.