If you're not able to connect to Plaxo website, navigate to Start > Control Panel > Network and Internet > Internet Options and open the Security tab. Click the Trusted sites icon and then press Sites. Type
http://www.plaxo.com/ in the first field and press Add. Click Apply and then OK. Also, launch your browser, press Shift + Ctrl + Delete key combination and delete all cache, cookies and temporary Internet files. If it doesn't help, I suggest you temporarily disable your security program and Windows Firewall.