No, the Paypal eCommerce toolkit is not supported on Dreamweaver CS5.
the System Requirements as listed on the Product detail page for the Paypal eCommerce Toolkit are:
Windows Vista, Windows XP, or Windows Server 2003
Dreamweaver CS4, Dreamweaver CS3, Dreamweaver 8
Works with static and dynamic pages
Payment Service
PayPal Account
Macintosh
Mac Power PC 10.4.x, 10.5.x
Mac Intel 10.4.x,10.5.x
Dreamweaver CS4, Dreamweaver CS3, Dreamweaver 8
Works with static and dynamic pages