Let's say I want to add the Kraken and Giant Crab to Scorched Earth, this is how I would set up my game.ini file.

These are the various creature spawn regions in Scorched Earth. Some of them are self explanatory, others make no sense and are probably best left alone unless you're trying to remove a creature from wild spawning in anywhere you're unfamiliar with. I recommend you check the map on the ARK Wiki to be completely sure of the biome names:
http://ark.gamepedia.com/Scorched_Earth

SE_DinoSpawnEntries_DeathWorm
SE_DinoSpawNEntries_RexSolo
SE_DinoSpawnEntriesBadlands
SE_DinoSpawnEntriesCanyonCave
SE_DinoSpawnEntriesCanyons
SE_DinoSpawnEntriesDunes
SE_DinoSpawnEntriesEZMode
SE_DinoSpawnEntriesGreenDesert
SE_DinoSpawnEntriesMountain
SE_DinoSpawnEntriesMountainCave
SE_DinoSpawnEntriesOasis
SE_DinoSpawnEntriesRuinsCave
SE_DinoSpawnEntriesRuinsCaveNoGolem
SE_DinoSpawnEntriesSandworm
SE_DinoSpawnEntriesWater
SE_DinoSpawnEntriesWyvern
SE_DinoSpawnEntriesWyvernTrench

[Example inside here]

[/script/shootergame.shootergamemode]
ConfigAddNPCSpawnEntriesContainer=(NPCSpawnEntriesContainerClassString="SE_DinoSpawnEntriesMountain_C",NPCSpawnEntries=(
(AnEntryName="GiantCrab",EntryWeight=1.0,NPCsToSpawnStrings=("Uragaan_Character_BP_C")) ,
(AnEntryName="Kraken",EntryWeight=0.002,NPCsToSpawnStrings=("Kraken_Character_BP_C"))),
NPCSpawnLimits=(
(NPCClassString="GiantCrab_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.000015) ,
(NPCClassString="Kraken_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.000015)) )

[Example inside here]


[/script/shootergame.shootergamemode]

** This only gets added once to the top of your game.ini file. If you've ever made ini edits it will already be there (I'm not sure if procedurally generated maps also add it since they add code to this file). **


ConfigAddNPCSpawnEntriesContainer=

** This tells the game you're starting a new spawn region, use a new one for every new biome you set. **


(NPCSpawnEntriesContainerClassString="SE_DinoSpawnEntriesMountain_C",NPCSpawnEntries=(

** Replace the biome here with a biome of your choice. **


(NPCSpawnEntriesContainerClassString="SE_DinoSpawnEntriesMountain_C",NPCSpawnEntries=(
(AnEntryName="GiantCrab",EntryWeight=1.0,NPCsToSpawnStrings=("Uragaan_Character_BP_C")) ,
(AnEntryName="Kraken",EntryWeight=0.002,NPCsToSpawnStrings=("Kraken_Character_BP_C"))),


** Replace the creature IDs with the creature(s) of your choice, or copy paste more of the GiantCrab line to add more creatures to a spawn region.

The entry weight is a somewhat sketchy thing to edit, I've had sucess with 0.002 making creatures genuinely rare, and 1.0 will make them fairly common.

Keep track of the brackets, all the creatures up until the last one will end in two brackets, but the last needs to end in three. **


NPCSpawnLimits=(
(NPCClassString="GiantCrab_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.001) ,
(NPCClassString="Kraken_Character_BP_C",MaxPercentageOfDesiredNumToAllow=0.001)) )

** This number value is intended to represent the percentage of the chosen biomes population this creature is allowed to fill, but I've yet to have any success with it.

Again, watch the brackets carefully, they all end in two brackets until the last creature which will end in three (with a space before the last bracket). **


** You can remove a creature from spawning naturally by adding these lines (replace the creatures ID with the one from its spawn code). **


DinoSpawnWeightMultipliers=(DinoNameTag=Kraken_Character_BP_C[,SpawnWeightMultiplier=0][,OverrideSpawnLimitPercentage=True][,SpawnLimitPercentage=0])