The following has been reported to work for an iMac (running Tiger 10.4.4) having problems getting Myst IV to run. No guarantee it'll work (or work with Myst V as well), but it's worth a try.
Get Info on Myst4.app and set it to Read/Write
Open Terminal
Type: cd_ (that's "cd" followed by a space, no quotes)
Drag the Myst4.app icon into the Terminal window, which will give you the path to the app
Press return.
Type the magic line: chmod a+x Myst4
Press return, quit Terminal.
Now, instead of going into the contents to run the modified file, just double click on the Myst4.app icon.