Have you changed your XCode defaults? I'm using 3.1.4 and I don't have to hit page up. Here are my exact key presses:
I can also hit escape first and start typing the macro name.
Edit: Argh! Your demon has infiltrated my XCode! ;) If I auto-complete some things, they become sticky as the new starting "a" macro--I can't even backspace and type "a" to use the first "a" macro. Each file has it's own persistent memory too, so switching files changes the default. Using a method or property or any other parsed symbol seems to have priority over the macros so the default will never go back to the initial "a" macro once you auto-complete a parsed symbol.
Pretty weird bug. I must not have seen it because I don't use macros much.
The only way I found to fix this is to change the "a" macro to some other auto-complete such as "ai". That way when you type "a" "escape", typing "i" next will select the macro you want even if there's a bad sticky default.
Start with this reference http://mcdevzone.com/2009/04/09/useful-xcode-tricks/
Then edit "~/Library/Application Support/Developer/Shared/Xcode/Specifications/TextMacros.xctxtmacro/Contents/Resources/ObjectiveC.xctxtmacro" and change the "CompletionPrefix = a;" to some other prefix.
When you restart XCode, the new prefix will work in place of the old "a" macro.