You need to use a font that supports it, and some text selected, then it will not be greyed out.
To find a font that works within your existing font set quickly, try the following:
1. Open the text+font dialog, third tab, unfold all parts
2. Click on the canvas with the text tool, type a letter
3. Click into the font dropdown at the left above the canvas
4. Use the arrow keys to quickly change the fonts
5. Watch the options change in the font variants tab