If you have an iPhone running iOS 4 (or newer) or an Android phone running Android OS 2.1 (or newer), accessing Google+ is a matter of downloading and installing the appropriate free app. However, there is no app for an iPhone running an older version of iOS, an Android phone running an older version of Android or some other phone (Nokia with Symbian, Windows Mobile or Windows Phone). If your phone is in this last category, Google has a support page to help you out.
Google Mobile Google+ support
There are two mobile web access methods for app-less phones. Google describes the features available in the Android app, iPhone app and the two mobile web access methods on this support page.
The Web app is the more advanced of the two mobile web access methods. It is simple called Mobile on the Google+ page. The simpler Basic web app is called Basic mobile on Google+. Google recommends the Basic web app for BlackBerry, Nokia/Symbian and Windows Mobile. This basic access method also works with Windows Phone. Windows Phone 7.5 has an HTML5 compliant mobile browser that is not, unfortunately, fully supported by the Google+ Web app. You may be able to note the rendering errors in the screenshot here.