We have found with some free wordpress plugins, they have started to generate errors when added and activated within wordpress. The error will show when the plugin is activated and will state “Plugin Does Not Have a Valid Header”.
The reason is very simple. While most plugin developers zip their plugin folder with any extra txt files required, some other developers are putting the plugin folder within another folder. This second practice causes a problem as the plugin should consist of a folder with files and other folders inside, creating a second incorrectly named folder causes wordpress to look for the wrong files and thus assume that the header is missing or invalid.
You should download the plugin to your pc, extract it and check to see if it does have one folder directly inside the other. Login via ftp and delete the folder that was created when you added the plugin within your blog. Finally, upload the correct folder from your pc, login to your blog and click on activate next to the plugin you uploaded.
An example of this is “TubePress.NET 3.0.1″. This plugin folder has been placed inside a folder which cause the header error when it is activated. By downloading it and only uploading the correct folder it stopped the error from being displayed because the folder that the plugin was in had the correct name. Plugin folders will normally contain a file with an identical name to the folder. When this folder is placed within another folder, it loses the ability to work and be recognised by wordpress.