the dccallow feature is imposed by some networks to give permission to a specific user to send certain file types to you. (not quite the same thing as /dcc trust)
You arent doing anything wrong if you see the dialog popping up asking you if you want to accept the file. By default, mIRC will pop up a dialog asking you if you want to accept the file, however if you select the Auto-get file option then mIRC will automatically accept the file. (view/options/dcc/) You really do need to use caution tho if you use auto-get and with setting a trusted user to autoget.
