First, you shouldn't have to install Eclipse. I've heard of this happening before, but it doesn't have the folder structure that LeanFT's installer expects and causes issues.
You need to move the entire Eclipse folder to your local drive, such as C. Next, run the LeanFT installer, and point to the main Eclipse folder that you just copied and run the install.
As a side note, for the Java EE Eclipse version you are running has an addtional step for LeanFT 12.51. You need to download and install Google's GSON jar file and place it in the Eclipseplugins folder.
Then, restart Eclipse.