I am using the latest version for Mac OS. Accord to your FAQ, after selecting an item, I should "click "Snipe" on the toolbar"

There is no "Snipe" on my toolbar or anyplace else that I can see.

I can see a Snipe box but it says always "Snipe not activated". I can click the Snipe button, fill in an amount and it still says "Spike not activated".

How can I activate it?

It seems like you are reading the FAQ for Windows app.
If you run Mac OS app, there is no Snipe button. You just highlight the item you need, and you should see "Activate" checkbox on the bottom of the window.

Thanks. I have restarted the app and now there is a "save" button that has appeared on the bottom. Clicking the save button has solved the problem.