First Question: Anybody understand what it means in the Adept Focus where it talks about an adept focus raising the number of power an adept can have activated? The write up says an adept focus doesn't grant new powers so I am at a loss as to how this works.

Second Question: Has anyone played the Infusion metamagic? Has it proved useful? Do you have to leave some power points unallocated to work with Infusion or does it allow your power point total to temporarily increase above your magic rating? Assuming Infusion does allow your power point total to exceed your Magic rating would it be reasonable to drop the power point "burn out" that occurs after Infusion runs out if the character did leave enough power points unallocated to cover the amount received from Infusion? The character would stil have to make the drain save and I would still limit the max number of new powers that can be manifested per the write up.

You can only have a number of powers active as you have Magic. So if you have 6 Magic (starting), you can only have 6 powers active at any given time.
If you use an Adept Focus, that power(s) counts as one of the powers against your cap (=Magic Attribute). So 1 AF power takes 1, and 5 "natural" powers.

It was put in place in SR3 when you could have more Power Points than Magic Attribute.

Adept Foci add to your magic attribute with regards to powers used. They're specifically intended to be used in conjunction with Infusion foci and the infusion metamagic.

Infusion/Infusion foci take up magic points to use. And yes, infusion has been useful. Somatic Control and Cognition more so, though.

Adepts have a limit on 1.) how many power points worth of powers they can have active at once and 2.) how many levels a single power can have. This limit is equal to the characters Magic rating. However with the 20 karma = power point rule, and combinations of magic loss, geas , initiation etc, it is possible for an adept to have more power points worth or even levels of power than their current Magic attribute. An Adept Foci provides the increase in Magic rating, allowing more than the usual amount or powers to active and used at once.
Also some powers are activated/used by rolling the adept's Magic, thus Adept Foci also gives you more dice to roll on your powers.
That's the basics at least, I haven't looked at infusion foci enough to comment on those.