Since v0.18.0, the SME environment integrates a simple text editor to manipulate Signal Text files inside Eclipse. The current version provides only syntax highlighting for Signal keywords, for comments, and for constant value using primitive types (string, character, or numerical value).
To use this editor, you have only to double-click on the file with extension .SIG. Maybe the Signal Text editor is not selected by default, so, you have to right-click on the file and select Open With->Signal Text Editor.