Ensure that the latest version of CALC is on your iPhone and that your Apple Watch is paired properly

Next - open the “Apple Watch” app on your iPhone, scroll down to find CALC on the list - open it

Turn on “Show App on Apple Watch” and optionally also “Show in Glance” if you wish to see CALC's Watch History log in Glances

It will take a few seconds to sync onto the watch, you can see the progress by opening the Watch apps view (press the dial) and see the CALC icon appearing and getting loaded