Hi all,
I need to send fro pure data to an arduino a signal to light up a led.
How can I do that?

Have you checked the pduino library? I used it some years ago with satisfactory results, though I am certainly no expert on it.

For exchanging via usb you can use [comport]
https://puredata.info/community/pdwiki/ComPort

It's part of Purr Data and it works well.

Xaver

I've written a tutorial on the communication between Pd and the Arduino. You can find it here http://drymonitis.me/code/ it's called "Arduino for Pd'ers". The very first project is about blinking an LED controlled from Pd.

I highly recommend the above tutorial projects. Thank you for sharing them.