Can I make the form fields arrange horizontally?

Yes.  Using CSS, you can float your form elements left to right.  First, we float our form elements with

.locator_form {
border: 1px solid #EEE;
margin-bottom: 5px;
overflow: auto;
float: left;
}

Then ensure the form itself pushes the map down.

.com_locator_forms {
overflow: auto;
}

Now make sure the submit button is on its own line below the form elements.

.locator_submit {
clear: both;
float: left;
}