Try the following, locate the DLL named Microsoft.Office.Interop.Excel.dll for the version of Excel you are using, I found it in C:\Program Files (x86)\Microsoft Office\Office15\DCF on my machine (Windows 7 x64), it will be in a different location on a 32 bit machine.
Now go to the Add .Net Assembly Reference dialog in the Silk Test Workbench VB.Net script window, select the Browse button and locate the file and select it. When you go back to the Add .Net Assembly Reference dialog find the entry named Microsoft.Office.Interop.Excel select this and press OK. This will add the entry to the references.
Now import the assembly adding the following at the top of the script.
Then you can write something like this ...
It should be quite easy to translate sample C# code to VB.Net for your purposes.
Dim xlApp As New Microsoft.Office.Interop.Excel._Application