Hi Claude, You have to transfer the entire iPhoto Library to continue to work on another Mac. You can find the iPhoto Library in the Pictures folder. There is NO direct way to export the greeting card or particular albums from iPhoto except to movie file or individual photos. It is the limitation of iPhoto. But I've few suggestions for you.
--- > If both Macs are connected to the same network, then you can share the albums with another iPhoto. Follow the instructions in here : http://support.apple.com/kb/PH2435
--- > If you copy this iPhoto Library to another Mac, then you can transfer the albums from this iPhoto Library to another library using a third party application named iPhoto Library Manager. Get it from here : http://www.fatcatsoftware.com/iplm/
Follow the instructions in here to transfer with iPhoto Library Manager : http://www.fatcatsoftware.com/iplm/Help/copying%20albums%20and%20events.html
--- > If you just want to copy the photos in the Albums, then create a folder in the desktop, then select the photos in the album, then click and drag those selected photos to the desktop. Then you can upload that folder to dropbox. Unfortunately, this method won't work for greeting card or slideshows.
I know this is not the answer you've expected or want to hear, but it is the limitation of iPhoto and that is how Apple designed it. Apple didn't make a way to save the Albums or greeting card/slideshows, so they can be added to another library and continue working on it.
Please let me know if you've any questions or would like any additional informations.