Sony Ericsson has no WTK for Mac OS X, so first download the Sony Ericsson WTK and install on a PC (that is if you want to work against their libs), then snatch all the jars and put in a folder on the Mac that you will be working on from now on.
Download and install Eclipse Classic SDK (or your preffered vesrion). (http://www.eclipse.org)
For Subversion support download and install Subclipse using the update menu in Eclipse. (http://subclipse.tigris.org)