I have looked through all of the keybindings under Preferences | General | Keys.
I have also looked under Preferences | Aptana | Editors.
All to no avail.
Does anybody know how to change the autocomplete key from Enter to Tab or something like that?
It depends on the version of Aptana Studio.
In Aptana Studio 2, Preferences > Editors > General: "Insert selected proposal with 'Tab' key..." sounds like it's what you are looking for.
In Aptana Studio 3, it's working that way automatically. "tab" and "enter" both work for for me to autocomplete.
Note, that means that "tab" and "enter" both work. There is no current way to only have "tab" on and "enter" off.
On a related topic - if you are annoyed by the autocomplete in Aptana 3 every time you hit 'tab', you can put a delay on the auto-insert popup (or disable it altogether).
Go to Aptana->Editors and turn off "insert single proposals automatically" and then turn the auto-display drop down to "delay 1 second" or "off".
I can't be the only person that gets bothered trying to format my code with 'tab' only to have Aptana 3 insert a random property or statement.