An update to the mobile apps allows you to search recipes on your device. This searches through the recipes in your MasterCook.com account that your app is accessing.
Open up My Recipes on the app or open a cookbook. You will then see the search text box at the bottom of the screen. This searches your recipe titles. If you want to find all of your recipes that have "sauce" in their name you would type in sauce for example.
There is also a filter that allows you to find recipes by category. Click on the Filter link to the right of the search text box to access it.