Zoom in/out keyboard shortcuts for Swedish keyboard?
Firstly, I'd check which keyboard shortcuts are documented for zoom in/out in the relevant menu.  My View menu (in Aperture 3.4.4) is as shown below: note the keyboard shortcuts to the right of the menu option.


I read that '=' is shift-0 on a Swedish keyboard.  On my UK keyboard, '=' has a key to itself (pressing shift-= produces '-'), so I'd guess that the issue is to do with the shift key.
So, secondly, a different approach -- if we can't work out how to generate this key combination on a Swedish keyboard, I'd suggest changing the keyboard shortcut.  To do this, go to Apple menu &gt; System Preferences &gt; Keyboard &gt; Keyboard Shortcuts.  Select Application Shortcuts on the left.  Press the '+' button, then select Aperture as the application, type Zoom In for Menu Title (making sure you get the spellling and capitalisation exactly right), and then type the key combination you'd like to use in the Keyboard Shortcut box.

Finally, if you are eligible for software support, I'd suggest calling AppleCare and asking them for help...