When you switch your alphanumeric bookmark, you can show the bookmark popup using SHIFT + F11 and enter the letter of the bookmark you want to go to. After you type the letter, it will immediately open the editor in the appropriate tab.
Looking at the list of commands in "Settings" / "Key Card", I see no way to assign a shortcut to specific alphanumeric bookmarks in the same way that it works for number-mnemonic bookmarks, so this is probably the only way.
I confirmed that this solution works on both Windows and Linux versions of IntelliJ 13. I hope it works the same on the Mac version.
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