Hello,
Please make sure you downloaded v3.0 build 1015 directly from our website, If you obtained it from another source it may not properly accept your registration key.
http://www.flashfxp.com/download.php
Also make sure that the flashfxp.key and all the other files in your FlashFXP directory aren't marked as read-only, this can occur if you backed up FlashFXP to a CD-rom and then copied it back to your computer.