Even though the original iPad is able to run the latest version of iMovie for iOS, Apple has decided to not allow original iPad users to install iMovie directly through the App Store. This decision has to do with the original iPad’s lack of cameras, slower processor, and less RAM.
Original iPad users who want to run iMovie on their original iPad shouldn’t fear as iMovie can work on their iPads… without a jailbreak.
Open the iPhone Configuration Utility, click applications on the left side and click open iMovie. Then connect your first generation iPad to your computer, and then click the install button in the configuration utility software. Now you should be good to go. Let us know how it works out for you, in the comments.