Sbt add plugin error

I've read all similar questions, but none of the suggestions fixes my problem. I may not see this.

The problem is that I have project / plugins.sbt and when I try to add eg

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.11.2")

      

        
        
        
      

    

it gives me

[error] (*:update) sbt.ResolveException: 
unresolved dependency: com.eed3si9n#sbt-assembly;0.11.2: not found

      

        
        
        
      

    

then add

resolvers += "sonatype-releases" at "https://oss.sonatype.org/content/repositories/releases/"

      

        
        
        
      

    

but the error is the same

my sbt version

sbt launcher version 0.13.8

      

        
        
        
      

    

and scala version

Scala code runner version 2.11.6 -- Copyright 2002-2013, LAMP/EPFL

      

        
        
        
      

    

Maybe someone can point me in the right direction. Thanks in advance.

EDIT: when I put

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.13.0")

      

        
        
        
      

    

for /assembly.sbt project I got the same error

Actually i dont think the error is for sbt-assembly it gives the same error when i try to add other plugins

[info] Resolving com.eed3si9n#sbt-assembly;0.13.0 ...
[warn]  module not found: com.eed3si9n#sbt-assembly;0.13.0
[warn] ==== local: tried
[warn]   .ivy2/local/com.eed3si9n/sbt-assembly/scala_2.11/sbt_0.13/0.13.0/ivys/ivy.xml
[warn] ==== public: tried
[warn]   https://repo1.maven.org/maven2/com/eed3si9n/sbt-assembly_2.11_0.13/0.13.0/sbt-assembly-0.13.0.pom
[warn] ==== Typesafe Repository: tried
[warn]   http://repo.typesafe.com/typesafe/releases/com/eed3si9n/sbt-assembly_2.11_0.13/0.13.0/sbt-assembly-0.13.0.pom
[info] Resolving jline#jline;2.12.1 ...
[warn]  ::::::::::::::::::::::::::::::::::::::::::::::
[warn]  ::          UNRESOLVED DEPENDENCIES         ::
[warn]  ::::::::::::::::::::::::::::::::::::::::::::::
[warn]  :: com.eed3si9n#sbt-assembly;0.13.0: not found
[warn]  ::::::::::::::::::::::::::::::::::::::::::::::
[warn] 
[warn]  Note: Some unresolved dependencies have extra attributes.  Check that these dependencies exist with the requested attributes.
[warn]      com.eed3si9n:sbt-assembly:0.13.0 (sbtVersion=0.13, scalaVersion=2.11)
[warn] 
[warn]  Note: Unresolved dependencies path:
[warn]      com.eed3si9n:sbt-assembly:0.13.0 (sbtVersion=0.13, scalaVersion=2.11) (assembly.sbt#L2-3)
1.0

      

        
        
        
      

    

EDIT

Good for others who are sloppy as I am in all the docs it says like project / plugins.sbt, or project / assembly.sbt etc ... Actually I thought that " project " should be the application project folder. but this is [your_project_name] / of the project :( so all the specified confs must be in this directory.

Thanks a lot for the answers.