There are no Lightroom plugins in the export dialog left column.
When I click the Plugin manager, LR tells med that the plugins are installed and working, but they do not show up in the export dialog. The actual plugin files are stored in the folder C:\Program Files\Adobe\Adobe Photoshop Lightroom 4.2, the program folder.
I am sorry, no, they are not - nowhere except in the program folder. Here is a screen shot of the Export dialog. OS is Windows 7, 64 bits, and everything else is working perfectly
Here is also a screen shot of the Plugin Manager
It shows the same for all, and I can locate the files in the spsified folder. This Plugin is checked for Reload for test purpose only.
The plugins you are showing in the the second screen capture are located in Library Module-Publish Services: Left hand side, bottom panel. - Not in the Export module.
The included plugins are designed to publish and manage photos with specific services like Flickr - not to export to Flickr. Jeffrey Freidl makes an excellent export plugin if you don't want to use the Lightroom Publish Services.