When adding a video url I get an error saying allow_url_fopen is not enabled
In version 4.1 and above you can add video by pasting a url from youtube or vimeo. When you save the url, the gallery will make a request to the video hosting site, and try to download the videos preview image. To do this the gallery needs the php.ini setting allow_url_fopen set to Yes. If this is not set then the image will not be downloaded an error saying "allow_url_fopen is not enabled in php.ini" will show.
In the short term, you can manually add video by uploading an image and copy and pasting the video embed code, please see this page for more info.
To see if the php.ini setting allow_url_fopen is set to yes, please go to the Joomla admin section, then system info, then php information. If allow_url_fopen is set to No, please ask your server admin to set it to yes, or edit your servers php.ini file and set it to yes.