How do I purge a hot water baseboard system? It seems to be filled with air and the water is not going through the baseboard.

Start furthest from the boiler and a bleed port should be on each radiator, open it till no more air comes out then move to the next furthest, repete till all radiators have been bleed,

This can take all day... be patient.

Ps: have a tray to catch the water and a towel ,sponge ,etc

Find the vent that is located highest in the system. That is where you start.