How do I log in?
On mobile, first open the menu using the button in the top-right corner. That will reveal the login link.
Alternatively, perform any action that requires you to log in, and then just tap the link that is shown in "Please login or register to [...]".
Note that some mobile devices, browsers or maybe even networks do not support the global auto-login feature. See Which browsers are officially supported? And what else do I need? or use the Network Login Troubleshooting page.