Syntax Error

I have a page on my site in which i wish to display links to brochures, and
i have stored an index of these links in my database.
however when i pass from the master page,
http://www.tripakltd.com/brochures.asp to the detail page, by selecting a
item from the list/menu and submitting, i cannot seem to be able to carry a
variable as a value, i.e i am submitting CategoryID as a variable so i want
it to equal % but i only seem to be able to get it to work if a specify a
figure, such as number 1, and then that isnt very useful because for every
product i select on the drop down list/menu i'll only receive the brochures
associated with number 1.