gofret, did u press "up" while booting up your psp with pandora ?
If u followed exophase tutorial, I would assume your MMS will be loaded with TM and DC. The default button in the setup with TM/DC is "up" when u first installed DC into your MMS.
If u have 2 DC installed, eg. the old one such as DC5, and then a new one such as DC7, u will need to change the config.txt at the root of your MMS to configure, for eg, "up" for DC7, "down" for DC5, depending on which CFW you want to install.
If that still doesnt help, then probably your mem stick may be a problem. You may wish to check if your mem stick is a fake using your psp --> magix gate supported (real), magix gate unknown (fake).
PSPident can tell the exact board version of your psp, google it. Crossextend could also does the same thing, which u can find in exophase.