'i am trying to install a disk ...' - what is on the disc? Have you indeed inserted the disc; but, are unable to open (launch, run) an application on the disc?
'... i have OS X so shouldn't i be able to install it???' - no, not if it is a 'System 9.x' or earlier application. Which translates to - non-MacOS X compatible.
-----
Locate the upper right most icon on your 'Desktop'. This is the boot hard disk drive, hdd. Double-click on the icon to open a folder window, displaying its contents. Is there a folder titled 'System Folder'? - if not, you do not have 'System 9.x' installed. 'Classic' is just a 'System Preferences' utility that launches (and cripples the functionality of) 'System 9.x'.
If the inserted disc does mount (is displayed on the 'Desktop') and there is no 'System Folder' folder present (on the boot hdd); and, a 'System 9.x' or earlier application (on the mounted disc) is being double-clicked on - do not expect MacOS X to open (launch, run) the application.