kotlin extension method access in another kt

Make sure your extension function is a top level function, and isn't nested in a class - otherwise it will be a member extension, which is only accessible inside the class that it's in:

package pckg1

fun String.add1(): String {
    return this + "1"
}

Then, if your usage of it is in a different package, you have to import it like so (this should be suggested by the IDE as well):

package pckg2

import pckg1.add1

fun x() {
    var a = ""
    a.add1()
}

You can use the with-function to use a member extension outside the class where it was defined. Inside the lambda passed to with, this will refer to the instance of A you pass in. This will allow you to use extension functions defined inside A. Like this:

val a =  A()
val s = "Some string"
val result = with(a) {
    s.add1()
}
println(result) // Prints "Some string1"