in webassist/themes/PerfHorseShop/css/dark_form.css
the select css:
select {
width: 390px;
background-color:#FFF;
-moz-border-radius: 0 6px 6px 0px;
-webkit-border-radius: 0 6px 6px 0px;
border: 1px solid black;
border-radius: 0 6px 6px 0px;
padding: 5px 5px !important;
line-height: 2.1em;
height: 33px;
margin: 0 !important;
}
the height: 33px; is constraining the select list to be too small to show the scroll bar. remove the height.