You tried to download BBM 5.0 didn't you? I had that problem too. You'll need to sync your device up with Blackberry Desktop Mangaer. Once connected you can click on the Application Loader icon to view a list of all applications available for your phone. BBM 4.5 should be there, with the box unchecked, just check the box again and click "next" and it will restore it.
If that doesn't work you can use this link to get it back if Desktop Manager doesn't have it loaded anymore :http://na.blackberry.com/eng/services/devices/#tab_tab_update
Also, if you do in fact want to get BBM 5.0 you'll need to make sure you update your Curve to the latest verison: 4.5.0.138. This you can also do thru Blackberry Desktop Manager or with the link I provided. At that point you'll be able to successfully install BBM 5.0
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