Assuming that your database connection is correct and opened and rs("Name") is the name of country this should work for you:
<%
dim rsSelected
set con = Server.CreateObject("ADODB.Connection")
con.open "DRIVER={Microsoft Access Driver (*.mdb)}; DBQ=" & Server.MapPath("WebData/DB.mdb") & ";"
set rsUser = Server.CreateObject("ADODB.RecordSet")
set rsUser= con.Execute("Select top 1 country FROM Users WHERE UserName = '" & username & "'")
rsSelected=rsUser("country")
set rsUser=nothing
%>
<select name= "ddlCountries" id="ddlCountries">
<option value="-1"> choose
</option>
<%
set rs = Server.CreateObject("ADODB.RecordSet")
set rs = con.Execute("Select * FROM Countries where mode=true")
while not rs.eof
if rs("Name")=rsSelected then%>
<option selected value=<%=rs("PkId")%>><%=rs("Name")%></option>
<% else
%>
<option value=<%=rs("PkId")%>><%=rs("Name")%></option>
<%
end if
rs.movenext
wend
rs.close
set rs= nothing
Con.close%>
</select>
Just make sure that Select * FROM Users WHERE UserName = '" & username & "'" always return single record.I would put some safeguards in case if it does not return anything.