How to add a field in the dataabse with install function for a plugin I am creating.

[ZiNgA BuRgA]

The MySQL query is incorrect.

You need to use an ALTER TABLE query to modify table schema.
http://dev.mysql.com/doc/refman/5.1/en/alter-table.html

UPDATE queries change values in the table, not the table itself.

Depending on what you do, you may also need to update the forum cache after you perform the query.  From memory, it should be something like:

PHP Code:

$cache->update_forums();

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