Internet Explorer interation

I have an online form that I need to frequently populate from an excel spreadsheet, on the first page is a drop down box where I would normally select a category from. Why I click the category using the mouse it automatically advances me through to the next page, however when I use the macro below to do it, whilst the box shows the correct category, the page is not advanced. Can you help me?

Sub DropDown()

    Dim ie As InternetExplorer

    Set ie = New InternetExplorer
    ie.Visible = True
    ie.Navigate "http://MYSITE/choose_cat.php"

    Do Until ie.ReadyState = READYSTATE_COMPLETE
    Loop

    ie.document.forms(1).Item("catid").Value = "28"
   
End Sub

As a bit of further information this is how the website deals with the box being changed (I don't understand it!):

<select name="catid" onchange="checkSelectedOption(this.value, document.add_ad,'Categories marked with *.');">