My group is just now getting into Shadowrun fourth edition. My friend found some great cheat sheets to make combat go more smoothly. I saw on the spellcasting cheat sheet that I could add my spellcasting focus bonus to resist spell drain. Problem is that I can't find that rule anywhere in the source book. Any advice is appreciated here. Thanks.

Originally, this was allowed use of the Spellcasting Focus, but it has been removed in the latest errata and in the SR4A printing.

Note:
The spellcasting focus before errata was (poorly) worded to say that the dice granted could be used for EITHER the spellcasting test OR the drain test, but not both.

I'm not trying to

So I'm trying to find this in the erata: (http://shadowrun4.com/resources/sr4a/sr4a_changes.pdf) and I'm just not seeing it. If I use "spellcasting by the numbers" drain is a part (part 6) of casting a spell. So why wouldn't a spell casting focus allow the dice used to resist drain rather than on the casting roll?

It's not part of the SR4A changes. Its's SR4 Errata.

http://www.shadowrun4.com/resources/sr4/sr4_errata_v18.pdf

Page 2, second column.