The APKBuilder (and many others) won't show up unless you are Premium. Please make sure that you are using the same email address in the Google Play app as before to retain your Premium membership.
If you manually install Premium plugins, they will install ok, but won't actually work.
Note: Sometimes your premium status can be a bit slow to check if you have a weak connection to Google's servers. In which case, switching to wifi and restarting the app can often help.
If you don't have Google Play installed or you don't want to pay monthly for Premium, then you can request a 'lifetime premium' code from support @
droidscript.org for $99