きれいに取得する方法が見つからなかったので、無理やり作ってみました。
VBA
1Function TableCells(tbl As Table) As Collection
2    
3    Dim dic 'As New Scripting.Dictionary
4    Set dic = CreateObject("Scripting.Dictionary")
5    
6    Set dic("1 1") =  tbl.Rows(1).Cells(1)
7    
8    Dim i, j, c
9    For i = 1 To tbl.Rows.Count
10    For j = 1 To tbl.Columns.Count
11        c = i & " " & j
12        If tbl.Rows(i).Cells(j).Selected = False Then
13            Set dic(c) = tbl.Rows(i).Cells(j)
14        End If
15        tbl.Rows(i).Cells(j).Select
16    Next j, i
17    
18    For j = 1 To tbl.Columns.Count
19    For i = 1 To tbl.Rows.Count
20        c = i & " " & j
21        If dic.Exists(c) And tbl.Rows(i).Cells(j).Selected Then
22            dic.Remove i & " " & j
23        End If
24        tbl.Rows(i).Cells(j).Select
25    Next i, j
26
27    Set TableCells = New Collection
28    For Each c In dic
29        TableCells.Add dic(c), c
30    Next
31
32End Function
VBA
1Sub sample()
2
3    Dim shp As Shape
4    Set shp = ActivePresentation.Slides(1).Shapes(1)
5    Dim tbl As Table
6    Set tbl = shp.Table
7
8    Dim coll As Collection
9    Set coll = TableCells(tbl)
10
11    Debug.Print coll.Count
12    
13    Dim c As Cell
14    For Each c In coll
15        Debug.Print c.Shape.TextFrame2.TextRange.Text
16    Next
17
18End Sub
<追記>
Excelから呼び出すのも同じ感じで行けると思います。
ただ、以下はPowerPointが既に起動している前提で書いているので、
そうでない場合はもう一工夫いるのかもしれません。
VBA
1Sub ExcelVBASample()
2
3    Dim ppap As PowerPoint.Application
4    Set ppap = GetObject(, "Powerpoint.Application")
5
6    Dim shp As PowerPoint.Shape
7    Set shp = ppap.ActivePresentation.Slides(1).Shapes(1)
8
9    Dim tbl As PowerPoint.Table
10    Set tbl = shp.Table
11
12    Dim coll As Collection
13    Set coll = TableCells(tbl)
14
15    Debug.Print coll.Count
16
17    Dim c As Cell
18    For Each c In coll
19        Debug.Print c.Shape.TextFrame2.TextRange.Text
20    Next
21
22End Sub
23