how do we activate bonjour on a pc?

I am assuming you have already done this: http://support.apple.com/kb/DL999

Note the following:

Use Windows Update to make sure you have the latest Service Pack installed for your computer.

Printer requirements

Bonjour Print Services works with either:

* network printers over Wi-Fi or Ethernet

* USB printers shared via a Mac or an AirPort base station

Firewall requirements

The Bonjour networking protocol sends and receives network packets on UDP port 5353. The Bonjour installer will configure the Windows firewall appropriately during installation on supported systems, but if you have a separate "personal firewall" enabled, you will need to ensure that UDP port 5353 is open for Bonjour to work correctly.

If you have installed everything, meet the requirements, etc... the service should be on all the time. If you want to see about turning it on and off:

"To stop and start the Bonjour System Service, use the Windows Services applet. Disabling Bonjour is effective until the next reboot, when it starts up again." - from http://support.apple.com/kb/ht2250

i have the message that the "bonjour" is not turn on in my pc.

In your Start menu, right-click Computer and select "Manage".

Expand "Services & Applications".

Open "Services". (Perhaps maximise the screen to better see what's going on.)

Right-click the Bonjour Service and select "Properties".

In the General tab, set the Startup type to "Automatic":

... and click OK.

Restart the PC and the Bonjour service should be running. If you still get the message from iTunes on the PC, perhaps try the following document:

iTunes for Windows: Bonjour alert due to firewall, security, or proxy software