I was getting this error today whilst trying to import my solution
Error while Importing a solution in CRM 2011 – The solution file is invalid. The compressed file must contain the following files at its root: solution.xml, customizations.xml, and [Content_Types].xml
after about an hour of frustration the solution to the not importing solution finally struck me.
The problem was I was zipping up the solution folder, this then creates a zip file with a folder at the root and then the three solution files
What you have to do is select the three xml files and then zip them up. This then doesn’t create the root folder.
This is definitely a “Monday Morning” error and it’s a pity CRM doesn’t have an root folder error which I’m guessing would save a lot of people a frustrating hour or so.