Firstly you need a 64-bit intel mac. Not sure if your imac is, but should be. Secondly you need OSX 10.5, if you wish to use LW 5. Otherwise, you will need to downgrade to LW 4.18. To do that, remove the install limewire.pkg file found in the Receipts folder found at OSX -> Library -> Receipts. Sample: Receipts folder sample image