Before e-mailing a purchase order you will need to have them enabled; to do that just click Settings > Purchase Order Options > Tick the box for Enabled and click Update.  To e-mail a purchase order to your supplier just go to Purchases > Select a Purchase > Click the E-Mail Purchase Order button.  You'll then be prompted to enter in the suppliers e-mail address (this automatically fills if you've got one set in the suppliers profile), a subject (this automatically fills with 'Purchase Order from [your company name]), message text.  When you're ready just press the Send Email button.