If you double click the files that you have downloaded either within Limewire or in the Finder, they will often open in Limewire as a document - which is obviously not what you want. You should do the following instead:

1. Open your shared folder in the Finder (look in the library window if you don't know where it is).

2. Drag .img or .dmg onto Disk Copy and .sit onto Stuffit Expander.

3. If the file unstuffs / mounts correctly without errors , open the folder that has appeared on the desktop and click on the installer. I would be very surprised if an installer file opened in Limewire as it should be an application, not a data file and would have no relation with LW.