HTC Thunderbolt is one of the toughest device to root. The XDA developers team member dbzfanatic has developed a utility Easyroot + S-Off that makes Thunderbolt root process very easy. This will only work with firmware versions lower than version 1.12.605.9.
Many android apps do not make it to Android marketplace. Still those are released by android developers in APK format and called off-market apps. To install a non-market app on Android phone follow the instructions below.