Q: how to download non-Apple store apps on iMac 10.8

You can download an app from the any developer site and then install it on your mac.

I use Diskaid to move files between iPd/iPhone and Mac, I can move every file including photos and music.

http://www.digidna.net/diskaid.

If your Mac is not allowing you to download 3rd party apps that are from a source other than the Mac App Store, it is likely the security setting in System Preferences. Go to Sys Prefs/Security & Privacy/General and set the download options best for you. You will likely find the download options grayed out. You will need to click the padlock below and supply your admin password to unlock the preference pane.

You can download what you want. Install. When you are not allowed to open, click it again while holding the CTL key, then click "open" then again click open.

You do not have to set the security items.

Apple's information about 3rd party apps and Gatekeeper is confusing. Some paragraphs refer to preventing installation, others refer to preventing apps from opening.

http://www.apple.com/support/osx/security/