I am running MAB v5.3.1.0 and changed the "Default Search Engine" to:
https://www.google.com/search?q=%s
I clicked Save and type in a search query: "test" and it took me to the following address in my default browser (Google Chrome), but I also tested against IE and Firefox and they both worked as well:
https://www.google.com/search?q=testPlease make sure you are running the latest version of MAB and if you still have the issues, please let me know your OS and Browser and I can take a look at the code to see if maybe it's pulling something from your registry that is overriding the default search engine entered.