In order to count keystrokes, Actimeter needs your permission to use your Mac's accessibility features. This is something that every Mac app needs to do (not just Actimeter), if it wants to access all keystrokes that you type.
Note that in Actimeter's case, this is entirely optional. Actimeter works just fine without these features. However, without accessibility features, all keystrokes will be ignored when Actimeter calculates your activity.
If you want Actimeter to use its full potential, you should grant access to accessibility features. This process is explained in detail below. Note that you need to do this after you've installed Actimeter; it can't be done in advance.
The wording and images below are for macOS Mojave 10.14, and for the English user interface. If you've chosen some other language for your Mac, the text might be in your own language. The windows and icons might also look a bit different, if you're using a different version of macOS.
Privacy statement: Actimeter never records, stores or transmits any user actions (keystrokes, clicks, cursor locations or mouse movements). It only counts them.
Step 1: Click the Apple icon in the upper-left corner and choose "System Preferences".
Step 2: Click "Security & Privacy" (3rd from right on the upper row).
Step 3: Click "Privacy" (the rightmost tab).
Step 4: Click "Accessibility" on the left-side list. You may have to scroll the list down to see it.
Step 5: Next, you need to authorize yourself to be able to make changes to your Mac's privacy settings. Click the orange lock icon in the bottom-left corner.
Step 6: Enter your username and password, and click "Unlock".
Step 7: Click the plus icon below the list of apps (the list may be empty at this point). This will open a file selection window.
Step 8: Next, you need to point out Actimeter from your Mac's hard drive. Go to your Applications folder, select Actimeter and click "Open".
Step 9: Now Actimeter should appear on the app list. Verify that the checkbox in front of the app's icon is checked. If not, click it.
Step 10: Click the red button in the upper-left corner to close System Preferences.
Step 11: Finally, you need to restart Actimeter: close it and then open it again. Actimeter is now able to count your keystrokes: when you type on your keyboard, the keystroke count in Actimeter's sidebar increases.