Try this out.
.aspx page
<script>
function DisplayCarrier(value) {
if (value == 5) {
document.getElementById('<%= cboCarrier.ClientID %>').style.visibility = 'visible';
}
else {
document.getElementById('<%= cboCarrier.ClientID %>').style.visibility = 'hidden';
}
}
</script>
<style>
.hidden { visibility:hidden; }
</style>
<asp:dropdownlist id="cboFileType" runat="server" CssClass="inputControl">
<asp:ListItem Value="1">Item 1</asp:ListItem>
<asp:ListItem Value="2">Item 2</asp:ListItem>
<asp:ListItem Value="3">Item 3</asp:ListItem>
<asp:ListItem Value="4">Item 4</asp:ListItem>
<asp:ListItem Value="5">Item 5</asp:ListItem>
<asp:ListItem Value="6">Item 6</asp:ListItem>
<asp:ListItem Value="7">Item 7</asp:ListItem>
</asp:dropdownlist>
<asp:dropdownlist id="cboCarrier" runat="server" CssClass="hidden">
<asp:ListItem Value="1">Test 1</asp:ListItem>
<asp:ListItem Value="2">Test 2</asp:ListItem>
<asp:ListItem Value="3">Test 3</asp:ListItem>
<asp:ListItem Value="4">Test 4</asp:ListItem>
<asp:ListItem Value="5">Test 5</asp:ListItem>
<asp:ListItem Value="6">Test 6</asp:ListItem>
<asp:ListItem Value="7">Test 7</asp:ListItem>
</asp:dropdownlist>
aspx.vb page
Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
cboFileType.Attributes.Add("onChange", "DisplayCarrier(this.value);")
End Sub