HELP can't drag/drop files into an encrypted disk image

TROUBLE!!! Just tried to create a disk image from a folder using the link "How to create a password-protected (entrypted) disk image" and following everything very carefully. I do see the .dmg in thevFinder folder, but can't drag and drop files into it even though I selected "read/write" when making the .dmg. [When I double click on the .dmg with no files in it, I do see a password requested popup.] What am I doing wrong?