It is hard to help without a site to test or even the source code. Some source code redirects to secure java so it can not be found. If they went to the trouble to require a userid and password, I doubt that they left source code open. If they did, this might get you started.
Sub Test_LoginYahoo() LoginYahoo ThisWorkbook.Worksheets("Main").Range("A1").Value2, _
ThisWorkbook.Worksheets("Main").Range("A2").Value2
End Sub
'Add Tools > References...
' Microsoft HTML Object Library
' Microsoft Internet Controls
Sub LoginYahoo(username As String, password As String)
Const strURL_c As String = "http://mail.yahoo.com"
Dim objIE As SHDocVw.InternetExplorer
Dim ieDoc As MSHTML.HTMLDocument
Dim tbxPwdFld As MSHTML.HTMLInputElement
Dim tbxUsrFld As MSHTML.HTMLInputElement
Dim btnSubmit As MSHTML.HTMLInputElement
Excel.Application.Cursor = xlWait
If InStr(username, "@") = 0 Then username = username & "@yahoo.com"
On Error GoTo Err_Hnd
'Create Internet Explorer Object
Set objIE = New SHDocVw.InternetExplorer
'Navigate the URL
objIE.Navigate strURL_c
objIE.Visible = False
'Wait for page to load
Do Until objIE.ReadyState = READYSTATE_COMPLETE: Loop
'Do While objIE.Busy Or objIE.ReadyState <> READYSTATE_COMPLETE
' DoEvents
'Loop
'Set document object
Set ieDoc = objIE.Document
ieDoc.getElementsByName("passwd").Item(0).Value = password
ieDoc.getElementsByName("username").Item(0).Value = username
ieDoc.forms("Login_form").Submit
Err_Hnd: '(Fail gracefully)
objIE.Visible = True
On Error GoTo 0
Excel.Application.Cursor = xlDefault
End Sub