How to export iPhoto slideshow from iPad to iMac or external drive?

You can publish the slideshow from your iPad as a webpage, and then download the webpage using a webbrowser on your Mac. This way, you can save it as a formatted slideshow:

See this Help page:

Publish slideshows on the web http://help.apple.com/iphoto/ipad/2.0/#blnk3d369745

Or, save the photos you used in the slideshow to your computer using iTunes: http://help.apple.com/iphoto/ipad/2.0/#blnkee26bc1f

Save photos to your computer using iTunes

• Connect your iOS device to your computer.
• Tap a photo, album, event, web journal, or slideshow and tap > iTunes.
• Tap Selected, or change the photos you want to save (if you are saving a slideshow skip to step 4):
  • Select different photos: Tap Choose Photos, tap one or more photos, and tap Next.
  • Select a range of photos: Tap Choose Photos, tap Range, tap the first and last photos in the range, and tap Next.
  • Select all the photos in an album or event: Tap All.
• In iTunes, click the button for your device and click Apps at the top of the window.
• Below File Sharing, select iPhoto (in Apps).
• Select the Shared Photos folder under iPhoto Documents.
• Click Save To and select the location on your computer where you want to save the items.
• To view your photos, go to the Finder and look in the location you selected above.