Librem.ONE Tunnel can be used on Windows 10. As Joao correctly points out, the native Windows 10 VPN (Start…Network Connections…VPN) does not currently support Tunnel (November 2019).
As Joao suggests along with the correct links, you must install the Community version of OpenVPN on Windows 10. (For new OpenVPN users, there are commercial and community versions of OpenVPN. I used the COMMUNITY version of the client.
Thus, as of November 2019, the steps to setup Tunnel on a Windows 10 platform are:
Download, verify, and install the community-version of OpenVPN client.
Open OpenVPN client. The first time you might get an error message. Just click cancel and close the client. Then re-open the client. You may get a message then saying that OpenVPN is already running.
Go to your system tray and right-click the OpenVPN icon.
Select IMPORT FILE… and import the certficate.ovpn file that you just downloaded from Librem.ONE. You should get an import successful message.
Now, the username and password for the OpenVPN client are the Tunnel-specific, long username and passphrase strings downloaded above. Enter these, not your normal Librem.ONE credentials, as the username and passphrase.
Go to system tray, right-click on the OpenVPN icon, and select CONNECT… . You may be prompted for your login credentials (again, use the Tunnel-specific materials and not your normal login). You should be connected via the Tunnel VPN (the OpenVPN system tray icon goes from an outline to filled with green). The OpenVPN client briefly displays the connection transaction in a pseudo terminal window.
Note: when connected via the VPN, I still have problems using my standard SMTP servers (even if they do NOT use port 25) and cannot send email. A temporary work-around for SMTP was to reconfigure (Thunderbird for example), to use the Librem.ONE SMTP servers instead.
The SMTP issue was quickly fixed by Purism. If you use your own SMTP server, Purism support simply must whitelist the service before SMTP works. SMTP now works fine.