I've found it's easier to get into v1.5.6 servers if you're just dying to play.
Basically all you have to do is almost identical to how you installed the v1.5.7 mod.
-Extract the v1.5.7 files to another folder, call it something like '@dayz1\Addons' in 'C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead' (or wherever you have it installed)
-Download the v1.5.6 files from http://cdn.armafiles.info/old/
-Extract those to the same place; 'C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead\@dayz1\Addons'
-Click 'Yes To All' when it asks if you want to overwrite the files.
-And change your 'Set Launch Options' in Steam to '-mod=@dayz1 -nosplash' and you're good to launch versions v1.5.6.
-If you want to change it back to launch v1.5.7 just change the launch options back to '-mod=@dayz -nosplash'.
Major differences between V1.5.6 - V1.5.7:
V1.5.6 less zombies with more simplistic path finding (They tend to wander less and are more predictable)
V1.5.7 has more zombies and zombies have improved path finding (They tend to wander more and aren't as predictable)