When I open up Fiji I  get this messege in a console window, 
[WARNING] Your ImageJ installation cannot be updated because it is read-only
I am running a Mac with the latest OS (Yosemite) 
Having issues using the image calculator etc...

Information about your version of Java:

  os.arch => x86_64
  os.name => Mac OS X
  os.version => 10.10.3
  java.version => 1.6.0_65
  java.vendor => Apple Inc.
  java.runtime.name => Java(TM) SE Runtime Environment
  java.runtime.version => 1.6.0_65-b14-466.1-11M4716
  java.vm.name => Java HotSpot(TM) 64-Bit Server VM
  java.vm.version => 20.65-b04-466.1
  java.vm.vendor => Apple Inc.
  java.vm.info => mixed mode
  java.awt.graphicsenv => apple.awt.CGraphicsEnvironment
  java.specification.name => Java Platform API Specification
  java.specification.version => 1.6
  sun.cpu.endian => little
  sun.desktop => null
  file.separator => /

The up-to-date check says: READ_ONLY

Information relevant to JAVA_HOME related problems:

  JAVA_HOME is set to: null
  imagej.dir => /Volumes/Fiji/Fiji.app

Information about the version of each plugin:

Activated update sites:
ImageJ: http://update.imagej.net/ (last check:20150501134914)
Fiji: http://fiji.sc/update/ (last check:20150509174919)