It is possible to install 1 or more Java versions on a computer. As such, it is possible to install both a 32-bit and a 64-bit version. Which one runs the Perforce QAC Eclipse Plugin depends on the version of Eclipse that is launched (see Eclipse Version section below).
If you only have one Java installation, you should be able to determine the Java version at a Windows/Linux command line by typing: java -version
Of the details this returns, you should see “64-Bit Server” included in the text somewhere. If not, then you are likely running a 32-bit Java environment.