Try the following:

Sub IndentTextInCell()
   Dim p As Paragraph
   If Selection.Cells.Count > 0 Then
       For Each p In Selection.Cells(1).Range.Paragraphs
           p.Indent
       Next p
   End If
End Sub

According to Word VBA Help, the Indent method is equivalent to
pressing the Increase Indent button on the Formatting toolbar.

If you want to set a specific indent, you'd have to use the LeftIndent
property instead -- something like this:

Sub IndentTextInCell2()
   If Selection.Cells.Count > 0 Then
       Selection.Cells(1).Range _
       .Paragraphs.Format.LeftIndent = 18  'Indent measured in
                                           'points.
   End If
End Sub

Note that setting a left indent for numbered paragraphs may cause a
conflict with the indent settings in the Numbering dialog box; the
latter will then eventually "win".