Make sure the SystemVerilog Perspective is open. You can do this by going to Window>Perspective>Show Perspective>Other>System Verilog.
Please refer to the Performance Optimization for some hints.
The most common cause for content assist to fail is an issue with the project setup that SVEditor used to identify files to be included in the project index. A file will get a blue dot on it's icon in the project explorer view, and in the file tab at the top of the editor when the file was included in the SVEditor index. If the blue dot does not appear: