Well, first you need to check if it works at all. You can see in poseidon prefs when enabling "track events" and "report current values" in hid/joypad prefs.

If it works and you enabled lowlevel emulation, then all you need to do with uae is to add :
joyport1=joy0 (or joy1,...) depending on the lowlevel unit number assigned to your joypad in poseidon.