I believe I am doing everything right. This is on a 10.10 iMac. I used the forge-1.8-18.104.22.1689-installer.jar to install the server and use the forge.jar file to start everything. The server starts fine and folks are able to play on the world. I give the server 1GB RAM and it never encroaches beyond 80% free.
I have tried to drop in (allegedly) forge-compatible mod (.jar or .zip) into the mods folder and, on forge server startup, always get the same error message in the server window:
FML has found a non-mod file XXXX.jar in your mods directory. It will now be injected into your classpath. This could severe stability issues, it should be removed if possible.
Of course, the mod is ignored by forge. Is there a versioning match that is failing? If so, is there a repository with links to forge compatible mods up to the latest version?
Also, as a second and very newbie question, do end-users have to install the client forge portion as well as the forge mods used by the server?