You can access the Lifts Calculator a number of ways in the btwb app.
Side Drawer Menu
Click into the side drawer menu from the home screen, then again on the Lifts Calculator menu item.
Links to the Lifts Calculator will also appear on movement screens. You can get to movement screens a number of different ways.
Click on the search icon in the top-right corner of the home screen. Click on the Movements tile.
Then use the search bar to find a movement, or select from the popular movement GIFs shown.
Using The Calendar
Find a programmed workout on your calendar. Click on the plus button to reveal the various actions, then click on "Prepare". Prepare takes you to the workout screen.
The workout screen contains a list of movements at the bottom. Click on a movement to visit the movement screen. The "+" will contain a link to the Lifts Calculator.