You most first disable the plugin
I'm running (a relatively fresh copy of) 3.4.1. I was able to select the same plug-in shown in your screen shot (Mylyn Bridge: Java Development) to get an enabled "Uninstall..." button. I'd suggest getting the latest updates and trying again.
There are distributions of Eclipse that come packaged with certain plugins that can't be mix-and-matched. That is the case with Mylyn, that went from a student's thesis project to an integral part of Eclipse. If you want to actually get rid of Mylyn, you may want to install Eclipse classic instead of the distribution you have, and then install the stuff you need.
The standard Eclipse distributions come with Myln.
BTW, Eclipser 3.5 completely changes all these screens :(
It sure looks like there's no way to do this.
I just had exactly this problem with a fresh download of the Eclipse Java IDE (Ganymede SR2). I didn't want Mylyn, but I didn't want "Eclipse Classic", either, because the download was twice the size, I presume because it includes source.
The apparent solution was to update everything and restart Eclipse, after which the "Uninstall..." button became enabled for the Mylyn plugins. Unfortunately, this only uninstalled the most recent, updated version (3.0.11 IIRC), reverting to the un-uninstallable 3.0.5.
"The Uninstall wizard allows you to review and uninstall items in your configuration. This wizard is shown when you select items and press Uninstall... from the Installed Software page. To uninstall software from your system:
Click Help > About and then click Installation Details... to open a dialog showing pages that provide more detail about your installation.
Click the Installed Software tab to see a list of the software items that you have installed into your system.
Select the items that you wish to uninstall.
Click Uninstall....
The Uninstall Details page will show you a list of the items that will be uninstalled. Expanding each item will show what additional items (if any) will be uninstalled as a result of your choice.
If you change your mind about which items should be uninstalled, you may click Back to see a checkmark list of the items you selected before. You may check and uncheck different items in the list, and click Next when you are ready to continue. Click Finish to start the uninstall.
Once all of the software is uninstalled successfully, you will be prompted to restart for the Workbench. Click Yes when asked to exit and restart the Workbench for the changes to take effect."
quote from eclipse help docs
eclipse 3.5(Galieo) Help>> Install New Software >> Already Installed Plugins >> select Plugins >> Uninstall works charm.
And just to update it for Helios - Eclipse 3.6
Help->Install New Software
at the bottom of dialog there is link What is already installed
once you click on that you will get list of installed plugins and Uninstal...
button
I just don't know what the hell these Eclipse developers are thinking! Removing plug-ins from Eclipse became a pain in the ass since Helios!