geologyhero
In more recent versions of iolite it will be in the Channels menu.
There is a very small chance that it has been disabled for some reason. You can go to Tools -> Plugins -> Manage and verify 1) that "Calculator" shows up in the list and 2) that its status is enabled.
I would recommend simply reinstalling the latest version of iolite. It is a built-in plugin, so a new copy will be put where it is supposed to be. The latest version can be downloaded from: https://iolite.xyz/download/latest.
All the best,