I installed Force IDE in my Eclipse. But all the code are opened in text editor. I tried open with > other editors and apex editor is still not there. Therefore I can't get the keywords highlighted or get the intelligent pop-ups. Is there any way I can resolve this?
It's been awhile since I've used Eclipse as my Force.com IDE, but I think you want to open the Force.com perspective.
The Force IDE does have an Apex editor.
If you go to "About" and click on the salesforce.com icon and then "Plug-in Details" you should see the "Force.com IDE Editors" plug-in listed.
The Apex editor is associated by file extension (so the "File Associations" should get set automatically on installation) and so should be available whatever the perspective:
so presumably the problem is that your installation has failed in some way.
Perhaps the Force IDE will recover some of its lost ground when eventually it moves to using the newer APIs - see Force.com IDE - any news on the "after the refactor version"?.