Recording external synth in logic ? Audio issue...?!

hello

I am having diffcutly with record audio into Logic 9

I am using an extreanl synth to record into Logic and when i press 'Record Enable" the RED light comes on and i can hear the audio from my synth BUT... when i press the space bar to play the beats and then try to record in the audio live from the synth i can not hear any signal from the external synth ..!?

Can anyone please advies how I can get this problem sorted ?