Changing Store Locator Address Search Placeholder is really easy, first of all you have to check which template you are using, you template can be Template 0, Template 1 or Template 2. Once you know the right template you have to modify its template file.
In this wiki tutorial, I'm using Template 0, which is default template of our plugin. Search for keyword "auto-complete-search" to get to that textbox which is used for our plugin search.
Add an additional attribute to that textbox with you desired text, in my image I've added "Want to Search a Place"
Once you add the placeholder attribute, save the Template file and refresh the Store Locator view.