When you try to install an application, you will be asked to provide the keychain password to confirm the installation on your computer. If you don't have an access to the keychain password, try to contact the person who created your username because he could also create a password for the keychain. Once you enter the keychain password, you will have the possibility to install the app. However, in most cases, it simply works by entering your username and password, then the installation will continue by itself.
