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It sounds to me like the library list is not showing on your screen.
The link below will take you to the User Guide for iMovie 10. In the section marked "If it's your first time using iMovie" there is a link that explains how to show the Libraries list in the sidebar. Once you have that turned on, you can access your iPhoto library.
Show or hide the Libraries list
- To show the Libraries list: Click the Show button on the left side of the toolbar.You can also choose Window > Show Libraries
- To hide the Libraries list: Click the Hide button on the left side of the toolbar.You can also choose Window > Hide Libraries
I hope this information helps ....