As far as I know, wherever you unzip/untar the download, it should be in directory .../sqldeveloper/sqldeveloper/bin. I have no idea if there is some other aspect of the installation process on the Mac that would put it somewhere else. Maybe something like:
I downloaded a zip file, but it only contains a single executable file. After I first executed it, it generated a folder called "system3.2.20.09.87" under /Users/<LOGNAME>/.sqldeveloper/. I was trying to find out which file under that folder that I can change to increase memory, but I couldn't figure out.