For supported platforms and known limitations please see http://eclipse.org/mylar/dl.php
The most likely reason is that you are using an incompatible VM (e.g. JDK 1.4), in which case you'll errors like the following in your log file. To check JDK version that Eclipse was launched with to to Help -> About Eclipse SDK -> Configuration Details and verify that the java.vm.version is 1.5. Mac users should refer to the last bug 1163477 for instructions on how to change the 1.4 default.
Root exception: java.lang.UnsupportedClassVersionError: org/eclipse/mylar/tasklist/MylarTasklistPlugin (Unsupported major.minor version 49.0)
This can also be the result of using an incompatible version of Eclipse. Separate versions and update sites exist for Eclipse 3.1 and for the latest 3.2 milestone), in which case you'll see errors like the following in your <workspace>/.metadata/.log file or in a Mylar view:
java.lang.NoSuchMethodError: org.eclipse.ui.internal.dialogs.FilteredTree.getFilterControl()Lorg/eclipse/swt/widgets/Text; org.osgi.framework.BundleException: The activator org.eclipse.mylar.java.MylarJavaPlugin for bundle org.eclipse.mylar.java is invalid
In this case the easiest thing to do is reinstall Mylar. Your tasks won't be lost, by default they are stored in the <workspace>/.mylar/tasklist.xml file which will be read next time Mylar starts. First, uninstall the old version of Mylar using Help -> Software Updates -> Manage Configuration. You need to first Disable on all Mylar features by right-clicking them, allow Eclipse to restart after the last is disabled, and after restart. Then ensure that the 3rd toolbar button is pressed (figure below) so that you see the disabled features to uninstall, and Uninstall them using the popup menu. If you don't uninstall the the Update Manager will think that you have the latest and tell you that there are no updates. Note that manually removing the plug-ins and features can lead to Eclipse configuration errors. After uninstalling update Eclipse by adding the correct update site specified at: http://eclipse.org/mylar/dl.php, and after that automatically or manually updating will install the correct version.
Either because Mylar failed to install or update, or because there was a problem reading the task list. By default Mylar keeps your task list in <workspace>/.mylar/tasklist.xml. If you move workspaces, and have not changed the Mylar data directory via the Task List preference page, the new location will be used when Eclipse restarts (hit Restore Defaults on that page to copy tasks back to the default location). If your tasks ever disappear please report this as a critical bug, and check the .mylar folder for a tasklist-backup.xml file, which will contain the previous state task list. Make sure to copy it somewhere it before adding any tasks or queries or restarting Eclipse because it will get overwritten on any modification.
If your workspace folder is not backed up it is a good idea to either backup the tasklist.xml file manually or periodically use File -> Import/Export -> Other -> Mylar Task Data. Note that when restoring an previous tasklist.xml file you should first close Eclipse.
The goal is for Mylar to have no noticeable effect on Eclipse's speed or memory usage, and any performance issue should be reported as a bug. The current performance profile is:
If you don't like Mylar's constant showing of Java members in the Package Explorer, select the drop-down menu, then "Filters..." and enable the "Mylar Java Declarations Filter". It will then stick in the menu in case you want to toggle between modes. Note that this will hide interest information about members that aren't in your current file (e.g. showing you which methods are landmarks) so this mode is not recommended, but can be useful for 1024x768 screen resolutions.
As you work and elements become landmarks (bold decoration), and these elements populate the Active Search and Active Hierarchy views. To force an element to populate the views manually make it a landmark by right-clicking or hitting Ctrl+Alt+Shift+Up Arrow.