What exactly are you looking to do?
If you are trying to clean up your iMac's hard drive.
Do a search for and download and install OmniDisk Sweeper and OnyX.
Here are some of my tips for deleting or archiving data off of your internal hard drive.
Have you emptied your iMac's Trash icon in the Dock?
If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Other things you can do to gain space.
Delete any old or no longer needed emails and/or archive older emails you want to save to disc, Flash drive/s or to ext. hard drive.
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive to disc, Flash drive or ext. hard drive and/or delete any old documents you no longer use or immediately need.
Uninstall apps that you no longer use. If the app has a dedicated uninstaller, use it to completely uninstall the app. If the app has no uninstaller, then just drag it to the OS X Trash icon and empty the Trash.
Also, if you save old downloaded .dmg application installer files, you can either archive and delete these or just delete the ones you think you'll never install, again.
Download an app called OnyX for your version of OS X.
When you install and launch it, let it do its thing initially, then go to the cleaning and maintenance tabs and run all of the processes in the tabs. Let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.
Typically, iTunes and iPhoto libraries are the biggest users of HD space.
If you have any other large folders of personal data or projects, these should be thinned out, moved, also, to the external hard drive and then either archived to disc, Flash drive or ext. hard drive and/or deleted off your internal hard drive.