There are 2 steps involved
Step 1 - Upgrade to Express with Advanced Features
Run the installer for SQL Express 2008 Advanced Features on your existing Express install. At the SQL Server Installation Center screen select
Maintenance, then choose
Edition Upgrade. Run through this, it will let you upgrade your Express install to Express with Advanced Features.
Step 2 - Add the Full Text Search Feature
After you have completed the edition upgrade, rerun the Express with Advanced Features installer. On the same screen as above, click
Installation and then choose
New SQL Server standalone installation or add features to an existing installation. Run through that and it will let you add Full Text Search to your newly upgraded installation.
Note: In Step 2, on the Installation Type screen make sure you choose
Add Features to an existing installation of SQL Server 2008 instead of
Perform a New Installation of SQL Server 2008.