If you're using some custom ROM, sometimes happens, that creator of such rom removed some files, which he/she understud as an obsolete.
Like
/system/bin/newsfs.msdos
This script is used for creating asec repositories.
Try to use one I've uploaded.
For example, most newcomming complainers about this problem comes from the group of Darky's 9.5 Extreme Edition ROM, in which this file is missing, and thus some apps, relying on asec repositories Angry Birds, Dolphin HD Browser Themes, and others, fail to install. My wife was one of them.
Just copy over this file (unrar first!) to the path I've noted, and be fine