I'm working on a brand new install of concrete5 version 8 on Ubuntu and ran into this error when trying to add extensions:

Unable to generate a marketplace token. Please ensure that allow_url_fopen is turned on, or that cURL is enabled on your server.

I eventually discovered that I needed to install the php curl extension, which I did like this:

sudo apt-get install php5-curl
sudo service apache2 restart

I hope that will help someone in the future.