Lets look at your hardware synth first.
There are 2 ways, one simple, the other can be a pain in the arse but that depends on the synth.
So I take it for granted your usb midi interface is connected, drivers installed and all is well.
First way: Take 2 midi leads. 'Out' of your synth and 'In' to midi interface. 'Out' of interface and 'In' to synth. Now in Logic create a new channel and make it midi channel 1. On the left hand side of the screen you should see your midi interface written there. If not click on that area and drag up/down and select your midi interface. On your hardware synth make sure it's sending on midi channel 1 and your set.
If you program using the matrix editor, when you click on a note you should hear your synth.
If you are wanting to trigger lots of sounds from your synth then this is the second way:
Select the multi feature on your hardware synth and select the sounds you want to use on say channels 1,2,3,4. (consult manual) Now in Logic repeat all steps and create more midi channels 1,2,3,4 making sure the midi interface is selected on each one.
As I was saying, for me, this creates more problems but give it a go.
Hope this has helped, anything that doesn't make sense, I appologize, and let me know
Jason Are
www.templeoftears.com