In the alpha version I have installed, once you activate the Akismet plugin, you get a menu option “Akismet Configuration” and that is where you put the key. Before you activate the plugin, you can’t see the Akismet Configuration tab to even enter the key. I seem to recall the 0.9 version acting the same way (you need to activate the plugin before you can even enter the key.)
When you say “went to activate it”: what is it?
What versions did you integrate?