First off, the 'myui' directory name is just a placeholder (you can name it anything you want).
I don't remember right offhand what the directory structure looks like when wanting to use a custom UI along with EQ2Maps. However, if I was in your shoes, I would....
1) Delete your current UI folder and start from scratch.
2) Install whatever custom UI you are wanting to use (in the myui directory you created, or some of the full UIs offered here will create a directory within the EverQuest II\UI subdirectory upon extracting the .zip file. If this is the case, make sure you don't end up with something like %Systemroot%\Everquest II\UI\myui\XXXX (XXXX being the name of the UI).
3) Re-run the EQ2Map updater and point it to the desired location of the UI you want to use. This will download the map files and place them where they need to be (whether it be in the base custom UI directory you created or an EQ2Map subdirectory of that customUI directory, or a combonation of both).
***PLEASE NOTE***: When you are done doing this, make sure your eq2.ini file is still pointing to the correct UI directory (assuming the name of the custom UI directory is changed through doing this), otherwise you will be stuck with the default UI and no EQ2Maps.