http://androidforums.com/avail-all-things-root/566407-root-zte-z990g-merit-avail-variant.html
you will need to follow this page for rooting.
But here is a one click script based off the same how 2 as above without all the manual copy paste. It may not work because you ran the other root script. I would factory reset 1st then run the script to see.
[Q] How do I root the AT&T Avail (ZTE Z990)? - Page 2 - xda-developers
Name of the script is called Z990.rar middle of the page.