I followed the steps from
http://androidforums.com/avail-all-things-root/566407-root-zte-z990g-merit-avail-variant.html,
I got to the part where it says:
Once you have rebooted, remount, install su.
Code:
adb remount adb push su /system/xbin/su adb shell chown 0.0 /system/xbin/su adb shell chmod 06755 /system/xbin/su
When I type adb push su /system/xbin/su,
I get "Cannot stat 'su': no such file or folder