How to enable inline JavaScript autocomplete in Eclipse IDE with the Aptana extension
See if you can go to Window > Preferences > Javascript > Editor > Content Assist and ensuring that, under Auto-Activation, Enable Auto Activation Is checked.
That should automatically bring up Content Assist in blocks in html files.
Tested on Vanilla Eclipse PHP development install.
If that is still not working, try pressing CTRL + SPACE in the script block and see if content assist pops up. That is what should appear when Auto Activation is checked.