You must replace fireball_particle with the name of your .PCF file.
Now go into hammer and load your map. Put a info_particle_system entity where you want your particle in the map.
In the properties, in the Particle System Name field, type in the name of the particle system you made, not the name of the .PCF file, but the name of the particle file you typed in when you pressed Create in the Particle Editor. In my case, its fireball_main.
Change "Start Active?" to Yes, if you want the particle to start emitting when the map starts.
If you've done everything right, you should have your custom particle ingame working like a charm.