I was having the same problem after a fresh install of Eclipse and Aptana.
What fixed it for me was very close to what Jason mentioned in his Answer. I checked the available software sites in Eclipse --> Preferences --> Install/Update --> Available Software Sites.
In this window there was an option for Aptana that was checked. I had given it a name of "Aptana" previously and the location was "http://download.aptana.com/studio3/plugin/install".
There was a second location listed for Aptana at "http://download.aptana.com/studio3/plugin/update/" but this option was not checked nor did it have a name. I gave it the name "Aptana Update" and I put a check in the box. I then restarted Eclipse and the error went away.
Let me note that I am using:
Eclipse Standard/SDK
Version: Kepler Service Release 1
Build id: 20130919-0819
AND
Aptana Studio 3
Version: 3.4.2.201308081736-7W7I57boG98RAi489ctbvKi7VXbq