Make sure the configs match.
lucas1998, would you mind adding this ^^ to the O-EAQ? It may not be the solution to this particular problem, but it could be for many SMP issues.
On a mac, you can't actually delete that folder by normal means. Unzip your jar, then use the command line zip utility to rezip your jar. Don't the folder option.
Forge never re-assigns block or item ID's. It only auto-assigns a block or item ID when mod is creating a new config file. If Forge finds a block ID conflict with pre-existing configs, it will crash.
Slot 204 is already occupied by forestry.energy.gadgets.BlockEngine@7657d63 when adding portalgun.common.block.BlockPortalMod@180af260
Change the configs of either Forestry or PortalGun, whichever one you installed last.
How is it possible then that FML was able to find and load a zip file located in your mods folder?
2012-11-23 20:33:26 [ForgeModLoader] Searching C:\Users\Mito\AppData\Roaming\.minecraft\mods for mods
2012-11-23 20:33:26 [FINE] [ForgeModLoader] Found a candidate zip or jar file mod_MobTalker.zip