That's a trivial, change. Really!
Code:
Private Sub CommandButton1_Click()
Dim mySynObj As Object
Dim SList As Variant
Dim i As Long
StrOut As String
Selection.MoveLeft Unit:=wdWord, Count:=1
Selection.MoveRight Unit:=wdWord, Count:=1, Extend:=wdExtend
Set mySynObj = Selection.Range.SynonymInfo
SList = mySynObj.SynonymList(1)
For i = 1 To UBound(mySynObj.SynonymList(1))
StrOut = StrOut & vbCr & SList(i)
Next i
MsgBox "Synonyms for " & mySynObj.Word & " are:" & StrOut
End Sub
PS: When posting code, please use the code tags, indicated by the # button on the posting menu, and structured code. Without both, your code loses much of whatever structure it had.