Q: I've a basic question when you encrypt any SQL stored procedure with SQL Shield there is no way to decrypt it back ? let say if you want to modify the stored procedure how would you do that ?

The behavior of the stored procedure is the same like with the native encryption. Once you encrypted the procedure you can not edit it anymore. Only delete and restore can be applied. Of course, every software can be sooner or later cracked. The only difference is that unlike the Microsoft we can quickly upgrade the software to prevent decryption.

Q: Will SQL Shield work with MSDE? Yes, it works with MSDE just like with SQL 2000

Q: Can you tell us what are the exact limitations of the free versions? Free version of SQL Shield is only limited by the source size of the stored procedure. Not longer than 555 chars.

Q: Could you tell more detail about redistribution license ? How it is installed , how to distribute the database ? Do I need install software on my customer's computer ? SQL Shield must be installed on every computer where encrypted procedures must run.

There are two possible ways:
You install SQL Shield at the client server and run the script which creates all database schema. Procedures marked with "WITH /*sqlshield*/ ENCRYPTION" will be encrypted using SQL Shield encryption.

The second way is to install it at your server, create encrypted procedures and then deliver database to the customer. But before attaching database you must install SQL Shield at the client's server.
Take a look at the example how SQL Shield works http://www.sql-shield.com/example.html

SQL Shield is licensed on per-server basis. This means the license must be purchased for every SQL Server where you install our software. Redistribution license allows you to install SQL Shield to your customers as a part of your own software without paying license for every server.