Note that you must also have 'Per Unit' pricing turned on, as the price slider will not work with the standard pricing scheme (i.e. hourly, daily, weekly, or monthly listings).
To turn on your pricing slider:
1) Log in to your admin dashboard, and navigate to Manage > Service Types
2) Select "Search" on the Service Type you would like to add a pricing slider to
3) Once in the "Search" section, scroll down to Price Filters and select "Show price slider".
5) Navigate back to your marketplace search results page, and you should find your price slider among the other filters at the top