Click on Preferences--->Behavior (under interface)--->Preferred Output Format
You might want to consider AZW3 instead of Mobi especially if any of your ePub's have any advanced formatting or embedded fonts you want to keep. Provided the Kindle you're using isn't an old Kindle 1 or Kindle 2 it will support AZW3/KF8. Not that Mobi isn't fine for most regular narrative style books.
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