This is a fairly common problem that can be easily overcome. To download the application, visit the following website from your phone :
mini.opera.com. By accessing it, the site will automatically recognize your phone and provide for a compatible version to download. If your device isn't recognized, select Other Download options > Code-signing Certificate. From there, you should try all the available options in the following order : "All Certificates", "Verisign", "Thawte", "no certificates". You should be able to install the application.