When you click on the MerlinXL vsto installation link you get the error
Exception reading manifest from
Old and out of date .NET version on the the client pc.
Old or out of date VSTO office runtime
The Security Certificated from Thawte used to sign the code was created using .NET4.5 which uses a new security algorithm. Unfortunately this algorithm is not support by older versions of .NET !!
Ensure .NET 4.5 is installed on the pc
Download and install VSTO office runtime from here
For Excel 2007 on Merlin v2.x if the MerlinXL homepage has an alternative 2007 installation link beneath the original vsto link try this. However due to expiration dates on security certificates this can only install v220.127.116.11 of the vsto.
If there is no link contact Support@MerlinXL.com to arrange for the alternative 2007 installation to be set up on your server