How to properly include dependencies in R-package?

I was trying now for several hours to build a package in R and getting a bit desperate about how slowly I progress. I managed quite fast to build a package with no dependencies, everything works fine. Due to recommendations in several posts, I'm using R Studio, devtools and Roxygen2 (being on Windows). With dependencies, I get problems when I CHECK (e.g. with devtools::check() ):

checking dependencies in R code ... NOTE Namespace in Imports field not imported from: 'ggplot2' All declared Imports should be used. See the information on DESCRIPTION files in the chapter 'Creating R packages' of the 'Writing R Extensions' manual.

Furthermore, check() deletes the import(ggplot2) line in the NAMESPACE. If I do check(document=F), it gives an cryptic error about a digest package which is not loaded. I read 'Writing R Extensions' - 1.1.3 Package Dependencies and Hadley's Wiki concerning how to write packages, but couldn't solve my problem. DESCRIPTION and NAMESPACE files of other R packages from CRAN don't look different to mine (for my eyes)?

Question: What am I doing wrong? Sorry for such a basic question, but I am at a loss and most step-by-step tutorials I've seen so far stop before explaining dependencies.

So far, I have 3 files:
A DESCRIPTION:

Package: test
Type: Package
Title: Foo
Version: 1.0
Date: 2014-03-21
Author: Bar
Maintainer: Foo <bar@mail.com>
Description: Blubb
Imports:
    ggplot2
License: GPL-3

A NAMESPACE:

export(is.equal.null)
import(ggplot2)

A R-File:

#' Extension of compare to include NULLs
#'
#' Works as an extension to usual compare
#' Compares two basic objects which in addition to usual compare can be NULL
#' Intuitive output: TRUE if both are equal or NULL resp., FALSE if both are unequal or only one is NULL
#'
#' @param obj1 Basic object like \code{numeric, char, boolean, NULL}
#' @param obj2 Basic object like \code{numeric, char, boolean, NULL}
#' @keywords compare
#' @export
#' @examples
#' is.equal.null(5,5)  # TRUE
#' is.equal.null(5,NULL)  # FALSE
#' is.equal.null(NULL,NULL)  # TRUE
is.equal.null <- function(obj1, obj2) {
  # Small helper function to generalize comparison to comparison of NULL
  # returns TRUE if both are NULL, and FALSE if only one of the objects is NULL
  bool <- obj1==obj2 
  #qplot(obj1)
  if (length(bool)) return(bool)
  if (is.null(obj1) & is.null(obj2)) return(TRUE)
  return(FALSE)
}

Solution

You need to declare imports in two places:

The DESCRIPTION file. You should have a line similar to:
```
Imports: ggplot2, pkg1, pkg2
```
The NAMESPACE file. Here you declare the packages you need
```
import(ggplot2)
```
or to avoid namespace clashes
```
importFrom(ggplot2, geom_point)
```
You can get roxygen2 to maintain the NAMESPACE file using the @import and @importFrom tags.

In your example your DESCRIPTION file looks OK, but you haven't added the necessary functions to the NAMESPACE.