Howto combine and use TeXstudio with Skim
Download Skim from here: http://skim-app.sourceforge.net/ and install the App.
(Usually unpack and drag the app to the Application folder.
If not already done, install latest MacTeX and TeXstudio.
In TeXstudio goto Options -> Configure TeXstudio -> Commands -> External PDF Viewer and enter the path to Skim. (These steps are optional. If you have the PDF opened in Skim, any compiled changes should be seen immediately as Skim uses SyncTeX.)
If you have compiled a document, then clicking the PDF icon in the internal viewer opens Skim.
If SyncTeX is installed correctly and Skim is running, in any document that is compiled, changes should be seen instantly.
Forward/Inverse search should be done by simply CMD+click inside the PDF resp. the source file. If further configuration is needed see section 4.9 in the TeXstudio manual.
That's about it.