First make a new folder and put that .apk file (which you want to extract for getting the source code) into that folder. Now you have to change the name of that .apk file with extension .zip. Also ...

Purely Java Based Analyze the contents of the APK file Decompile and extract the contents of the APK file Decompile the DEX files to JAVA source files (.dex to .java) [ New feature in v2.0 ] Select ...