Traditionally, since the calculator is found within the core applications, the only way to reinstall it is to either (1) reload your operating system on the BB or (2) wipe your device. One thing you may want to check is that the icon is not hidden. On the main screen of your BB, hit the BB key and hit show all. This has happened to me a couple of times where it would hide while i have the device in my pocket. If you need assistance with reloading the BB software, shoot me a PM and i will send you the steps to reload it.