Using Qgis both 1.8.0 and 1.7.4, the slider does not show up after selecting features and click "simplify feature" botton.
Any special setting needed to show the slider?
The process for using the 'simplify features' button is to click the 'simplify features' button then click the feature you want to simplify.
See the bottom of page 69 in the 1.7 user guide
EDIT: Actually the user guide for that section is a bit confusing and probably the source of the problem.