Originally taken from [http://extjs.com/learn/Manual:Resources#Aptana_.2B_Spket_.2B_ExtJS]

# Download and install Aptana Studio (includes Eclipse).
# Download the [Aptana Ext JS 2.0 plugin | http://orsox.mocis.at/download.php?list.1] 
# Start Aptana and navigate the application menu to: Help → Software Updates → Find and Install... → Search for new features to install → New remote site...
# Name: "Spket", Url: "http://www.spket.com/update/"
# Restart Aptana
# Watch this Spket IDE Tutorial to see how to easily add Ext code assist (you can point it at the latest /src/ext.jsb to keep code assist up to date with the latest Ext version). The steps are basically:
## Window → Preferences → Spket → JavaScript Profiles → New
## Enter "ExtJS" and click OK
## Select "ExtJS" and click "Add Library", then choose "ExtJS" from the dropdown
## Select "ExtJS" and click "Add File", then choose the "ext.jsb" file in your "./ext-2.x/source" directory
## #  Set the new ExtJS profile as the default by selecting it an clicking the "Default" button on the right-hand side of the "JavaScript Profiles" dialog.
## Restart Aptana
## Create a new JS file and type: Ext. and you should get the Ext Code completion options.

NOTE: On Ubuntu x86-64, I had to get eclipse, install Aptana a plugin, and then go from there.

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[ExtJS | CategoryArchive.Computing.Web.JS.ExtJS] - 
[Eclipse | CategoryArchive.Computing.IDE.Eclipse]