Settings / Language & keyboard and make sure that the keyboard you installed is checked. However, if the keyboard won't show it won't give you a choice to choose another one. (There is usually a notification on the pull down that let's you change keyboards, but only if you have another one checked.)
Also, settings / apps. Scroll down to find Google Keyboard and tap it. Let it refresh and then tap the button "clear data". Then restart the phone.
See if the keyboard now works. If not, go back to settings / apps, Google keyboard and tap the button "uninstall updates". See if that helps.
The new keyboard installed is now working and it worked for a while during phone calls but it quit again today. The keyboard is nowhere to be found and now the home and back keys don't work during phone calls either. I'm not sure how to get it working again. Now the previous suggestions don't do anything at all. Please help. Is it something I'm doing? I don't have any new updates that I know about...