any ideas on what to use to catch porqupines in a live trap

Salted apples.
Porkies are easily guided if you know where they are coming from and funnel them in the door.

At first I misread the title and thought it said, "Pogey" bait! LOL

I have a few pieces of old rough cut pine lumber that I soak in salt bine.

Can they smell salt? Or do they have to approach and taste it?

Good question, I don't honestly know. But I have read several articles on porcupine becoming sodium deprived and that's why you see so many hit along the road in late winter and springtime.
When I was contacted about my first porcupine job client stated that it was chewing on the backdoor threshold. Onsite interview with client revealed that she always went barefooted around the house and back porch. Sweat and minerals were deposited on the threshold from being barefoot. I paired the salt with pine as they eat the bark in fall and winter.

I know that doesn't answer your question on if they can smell salt or not but rather how I came up with that concoction that seams to work for me

Porky's are famous for chewing up wood tool handles, has to be learned behavior.

Turpentine on a rag works in Alaska. They eat Spruce trees. Turpentine smells just like a cut tree. They come right after it. I think Fatdawg's board will work great too. I would add some Turpentine to the board. They do eat salt and get hit a lot of the roads for two reasons. One is they want the salt. The second is they are nocturnal. Slow moving dark object in the road that doesn't move gets run over.

It's the same smell that is in a Grouse during winter up here. They eat the Spruce needles. Smells just like Turpentine. We don't shoot them during winter because it's so strong flavored. A Spruce tree is like a Pine tree down there.

If I couldn't get some Turpentine, I would just hack off a couple limbs of Pine Tree and rough up the bark. Should work good enough. They also use habitual trails. We can see them go from cabin to cabin in a straight line through the snow. They chew on the cabins and sleep under them during the day for safety from the bears. Find his trail, put the trap right in the middle. Good luck.

positive set on the trail?
Never baited.
always looked for the den and set trails...

They sure like to chew on T-111 siding. They will chew as high as they can reach. like a beaver. Saw a few mountain resort type homes and cabins over the years that had some significant porky damage to their homes. I have seem them chew aluminum signs also in the big woods country.