I am not familiar with the app identified in your posting (Flexi), but if it is not an Adobe product then you probably need to address this issue either with folks in the Dreamweaver forum who might be familiar with it by chance of using it or with the developers of the app (possibly Extend Studio: http://www.extendstudio.com/support.html).
Ned Flexilayouts 2 is an app which is endorsed by adobe in the dreamweaver extensions app I have downloaded it through the extensions app it is one of the promoted apps to download. I have tried to get a response from extendstudio but no luck as yet. Does the error message not mean anything to you??
I don't think Adobe actually endorses any of the 3rd party extensions. You use them at your own risk.
Are you downloading the App from Adobe Exchange?