You need to do it like this (all the commands must be 4 spaces from the begin if you know hat i mean)
Groups 0 spaces
groupname: 2 spaces
default option 3 spaces
permissionsoption: 4 spaces
Commands: 4 spaces
inhertitance: 4 spaces
inheritancegroup:4 spaces
Info: 4 spaces!
Prefix/Suffix/Build: 6 spaces

groups:
  Default:
   default: true
    permissions:
    - essentials.back
    - essentials.balance
    - essentials.delhome
    - essentials.home
    - essentials.kits.tools
    - essentials.msg
    - essentials.pay
    - essentials.sethome
    - essentials.sethome.bed
    - essentials.signs.use.*
    - essentials.tpa
    - essentials.tpaccept
    - essentials.tpahere
    - essentials.tpdeny
    - essentials.warp
    - essentials.jail.allow.mail
    - bukkit.command.help
    - essentials.helpop
    - essentials.rules
    - essentials.spawn
    - essentials.jail.allow.help
    - essentials.jail.allow.helpop
    - essentials.jail.allow.rules
    - essentials.help
    - essentials.motd
    - modifyworld.*
    - plotme.use.auto
    - plotme.use.home
    - plotme.use.info
    - plotme.use.clear
    - plotme.use.add
    - plotme.use.remove
    - plotme.limit.1
    - sg.arena.join.*
    - sg.arena.vote
    - sg.lobby.join
    - lockette.user.create.*
    inheritance:
    - g:groupmanager_default
    info:
      prefix: '&7[Explorer]'
      build: true
      suffix: ''

     NOT LIKE THIS
groups:
  Default:
    default: true
    permissions:
   - essentials.back          <-------- 3 spaces here (must be 4)
    - essentials.balance
    - essentials.delhome
    - essentials.home
    - essentials.kits.tools
    - essentials.msg
    - essentials.pay
    - essentials.sethome
    - essentials.sethome.bed
    - essentials.signs.use.*
    - essentials.tpa
    - essentials.tpaccept
    - essentials.tpahere
    - essentials.tpdeny
    - essentials.warp
    - essentials.jail.allow.mail
    - bukkit.command.help
    - essentials.helpop
    - essentials.rules
    - essentials.spawn
    - essentials.jail.allow.help
    - essentials.jail.allow.helpop
    - essentials.jail.allow.rules
    - essentials.help
    - essentials.motd
    - modifyworld.*
    - plotme.use.auto
    - plotme.use.home
    - plotme.use.info
    - plotme.use.clear
    - plotme.use.add
    - plotme.use.remove
    - plotme.limit.1
    - sg.arena.join.*
    - sg.arena.vote
    - sg.lobby.join
    - lockette.user.create.*
                                           <-----------no space here
    inheritance:
   - g:groupmanager_default           <------------ 3 spaces (must be 4)
    info:
      prefix: '&7[Explorer]'
      build: true
      suffix: ''