This is a simple question with a simple answer.

1) Open DTS job in design mode.

2) Package -> Properties -> Global Variables tab

3) Add your variable and a default value

4) Open your data transform task (or wherever you have a SQL string)

5) Click Parameters and choose your global variable

6) ? is the parameter notation to use. Example:

WHERE

.field1 LIKE ?

I hope this helps!

-A