To hide the file key, just uncheck the "File key" checkbox and then copy the file's URL to the clipboard. You can then send the link to the file via email, and then send the key in an encrypted email or IM session.
Because files and folders are encrypted on Mega's servers, the site cannot supply thumbnail previews of images or video, offers no online streaming component for video and audio files.
Sharing a folder is a little different from sharing a single file, and you can share folders only with other Mega users. If you share a folder with someone who is not a Mega user, they must sign up for an account in order to access it.
To get started, right-click the folder and select "Sharing." You will then get a pop-up window where you can see a list of users with whom you have already shared the folder, and can add new users. To add someone, enter their email address and choose the level of access you want them to have, which ranges from read-only, read & write, or full access.
Once a folder is shared, the folder icon in your main file manager dashboard will have a hand underneath it to indicate it is a shared folder.
Folders that others have shared with you will appear in a list under your friend's name in the contacts section. Let's say Tom@gmail.com shares a file with you. The shared folder will appear under Tom's email address in your contacts section where you can, depending on your level of access, add or delete files. This could be a great feature for group collaboration on a set of files, or to share a photo folder with family and friends, but sharing is where Mega starts running into problems.
Bugs, bugs, and more bugs
I came across many bugs during my Mega tests even while using Chrome. The most obvious problem is an SSL encryption error that denies access to the site or specific pages in the site. A few times I couldn't even open the site's help page because of the SSL issue. You can usually fix the problem by refreshing the page once or several times, but in some cases you may have to close the page and start over.
File sharing is also problematic. Sending files and folders to others is no problem, but once someone receives the file, they may have problems accessing them. I shared a file with one of my dummy accounts for testing as well as to a colleague's account. In both cases the file was "Temporarily unavailable" for download, even up to an hour after the file landed in the other Mega account.