distribute an application

But in this case, when my client opens the database, Notes will ask him for the certificate of my company , won´t it ?

Where can I get your e-mail address ?

I would like to know how to hide the design ...

It will ask them if they want to cross-certify, if they say yes it makes an addition to their user id and they won't be prompted again.  If they say no it will prompt them everytime they open it (I don't believe there is a way around this, but I do far more development than I do admin/installation).

Sorry, I meant to include my email in there.  It is fanatic@aros.net - I find it easier to communicate through email than through the experts-exchange comments if you need additional clarification.  

Here is how to hide the design elements though.....

Make a copy of your database template (not a replica).  Select the copy, go to File/Database/Replace Design...
Select your original database from the template list (your original database will need to be an .ntf file of course).
Check the box marked Hide Formulas and Lotus Script.
Test the newly created database thoroughly (the hide formulas and lotus script sometimes does wierd things).
From then on you won't be able to look at the design elements in that database (nor will anyone else of course).  So you can then distribute replicas of that database to customers.

You said that to hide the design elements, I should select my original database from the template list. How do I transform my .nsf database to a .ntf ?

Change the filename to .ntf, and then in the database properties under the desgin tab, check the database is a template box and give it a template name.

It´s there a way to reverse it later ? I´d like to have the possibility to change the design if necessary .

You can always go back to your original template and make changes.  Only the new database that got its design from your original will have the design elements hidden.