The VSTO security model relies on certificates, the inclusion list, and lastly, the ClickOnce trust prompt. it looks like you aren't seeing a dialogue box that says "install or don't install"?
if not, then your inclusion list may be disabled. if the inclusion list is disabled, then the ClickOnce trust prompt will never appear. follow the instructions in How to: Configure Inclusion List Security to enable the inclusion list.
Windows security issue, enable ClickOnce Trust Prompt by following the below steps:
- Open the registry editor: Click Start, and then click Run.
- In the Open box, type Regedit, and then click OK.
- Find the following registry key: \HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\.NETFramework\Security\TrustManager\PromptingLevel
- Change the Internet subkey String Value to AuthenticodeRequired (or Enabled)
- Change the MyComputer subkey String Value to AuthenticodeRequired (or Enabled)
- Change the TrustedSites subkey String Value to AuthenticodeRequired (or Enabled)