How to install an OSX application from source
Well...you could just download the compiled version. Then unzip it and move it to your applications folder.

But if you want to compile it yourself, that's fine, too. Once you built in in XCode, the built product will be in the Development folder (or the Deployment folder, depending on which build configuration you chose). Be sure to build the correct target. To build the Platyups.app, you want the Platypus target.

Once you built the app, you can run it from right there. Or move it to your applications folder.