1 - Use the package and deployment wizard to package the app, and setup will
install it in a folder that is automatically trusted.  (Probably too late for
that in your case.)

2 - You could digitally sign the app before deploying and the customer could
accept the digital signature before opening the db.  (Expensive and has to be
renewed every year.)

3 - The customer could hack the registry.  (Scary thought.  Build a registry
script for them to double click on.)  Create these 3 registry keys:

1 - VBA warnings setting (DWORD):

HKEY_CURRENT_USERSOFTWAREMicrosoftOffice12.0AccessSecurity

VBAWarnings = 1

2 - Sandbox mode setting (DWORD):

HKEY_LOCAL_MACHINESOFTWAREMicrosoftJet4.0Engines

SandboxMode = 2

3 - Macro security setting, either for all users or current user (DWORD, 1 =
low level):

(For all users on pc)

HKEY_LOCAL_MACHINESOFTWAREMicrosoftOffice12.0AccessSecurity

Level = 1

-----------------------
(Or for single user on pc)

HKEY_CURRENT_USERSOFTWAREMicrosoftOffice12.0AccessSecurity

Level = 1