@capbobokc: The simple answer is...you don't turn on 3G.
The long answer is this: The Streak or any cellular device will automatically pick out the strongest signal from a particular tower and lock on to that signal. When you change towers, for example when driving, the Streak repeats the process. Thus you do not "turn on" 3G. While that answers your question, it's not the entire story. The carrier you have and the type of Streak you own play a part in this as well.
There are two different Dell Streaks available. One for use with AT&T's frequencies and one for T-Mobile frequencies. If the carrier and the Streak match, then you will get 3G or HSPA*. If the carrier and the Streak do not match, then you are stuck on EDGE.
So, to summarize:
- AT&T (Carrier) and AT&T Streak = 3G, HSPA
- T-Mobile (Carrier) and T-Mobile Streak = 3G (To the best of my knowledge T-Mobile does not have HSPA.)
- AT&T (Carrier) and T-Mobile Streak = EDGE
- T-Mobile (Carrier) and AT&T Streak = EDGE
If option 1 or option 2 are true, then you are either in an area with no 3G/HSPA service, or the tower is malfunctioning. In which case you want to call your carrier and have them service their tower.
If option 3 or 4 are true, you will never get 3G on the Streak.
*Disclaimer: This statement assumes the user is in a 3G coverage area and the nearest cellular tower functions correctly.