Here's a play which uses a sub to place the IM name entered by the user
within the source table's AIM col (Not sure it's possible to do this using
formulas)

An implemented sample is available at:
http://www.savefile.com/files/3744393
Place IM name within AIM col.xls

Source table assumed in A4:D30,
headers* in A4:D4, data from row21 to 30
*LastName, FirstName, Email Address, AIM

In B1 is a DV droplist to select LastName
[created via: Data > Validation, Allow: List,
Source:  =OFFSET($A$21,,,COUNTA($A$21:$A$30)) ]

In B2: =IF(B1="","",VLOOKUP(B$1,$A$21:$D$30,ROW(A1)+1,0))
B2 copied to B3.

B2 extracts the First Name, B4 extracts the Email Address

The IM name will be entered in B4

Over B5 is a forms button titled: "Place IM"
assigned with the sub PlaceIM below
(sub is placed in a regular module)

Clicking the button will then place the IM name input in B4
into the table's AIM col (in line corresponding to the email add in B3)

'-------
Sub PlaceIM()
Dim rng As Range, Email As Variant
   Set Email = Range("b3")
   Set rng = Range("c21:c30")
       If Email = "" Then Exit Sub
       For i = 1 To rng.Count
           If rng.Cells(i).Value = Email Then
           rng.Cells(i).Offset(0, 1) = Range("b4")
           End If
       Next i
End Sub
'-------

To implement the sub:
Press Alt + F11 to go to VBE
Click Insert > Module
Copy n paste the sub into the code window (whitespace on the right)

Press Alt + Q to get back to Excel

Draw a forms** button on the sheet, and assign the sub "PlaceIM" to it
**from the forms toolbar. Activate the toolbar via: View > Toolbars > Forms
Re-position / re-size the button to fit over cell B5 (Hold down Alt key)