Generate DITA-OT output doesn't produce anything (wrong version of Java?)

FrameMaker 2015: I am unable to generate anything from the "DITA > Generate DITA-OT output" menu command.

First problem was that trying any of the three output options in the Generate DITA-Output menu produced a command window with this error:

Unable to locate tools.jar. Expected to find it in c:\ProgramData\Adobe\CS5\lib\tools.jar.

Buildfile: c:\Program Files (x85)\Adobe\AdobeFrameMaker2015\fminit\ditafm\DITA-OT\build.xml

I had no c:\ProgramData\Adobe\CS5\lib folder, but found a "\lib\tools.jar" file in my Java directory, and copied the "lib" folder to the location that FM was looking for.

That at least made that error message go away, but produced a log file with this error:

BUILD FAILED

C:\Program Files (x86)\Adobe\AdobeFrameMaker2015\fminit\ditafm\DITA-OT\build.xml:113: The following error occurred while executing this line:

C:\Program Files (x86)\Adobe\AdobeFrameMaker2015\fminit\ditafm\DITA-OT\plugins\org.dita.base\build_init.xml:99: java.lang.UnsupportedClassVersionError: org/dita/dost/invoker/ExtensibleAntInvoker : Unsupported major.minor version 51.0

Does this mean that I have the wrong version of Java -- if so, which version should I have? Or does it mean something else?