Auto-complete's popup.el has been extracted to a separate library.

I will submit a pull request to el-get's auto-complete.rcp later, for
now this solves my problem.

packages.el

@@ -28,7 +28,24 @@
 ;;
 
 (setq el-get-sources
-      '((:name birds-of-paradise-plus-theme
+      '((:name auto-complete
+               :website "http://cx4a.org/software/auto-complete/"
+               :description "The most intelligent auto-completion extension."
+               :type git
+               :url "http://github.com/m2ym/auto-complete.git"
+               :depends popup
+               :load-path "."
+               :post-init (lambda ()
+                            (require 'auto-complete)
+                            (add-to-list 'ac-dictionary-directories (expand-file-name "dict" pdir))
+                            (require 'auto-complete-config)
+                            (ac-config-default)))
+        (:name popup
+               :type git
+               :description "Visual Popup Interface Library for Emacs"
+               :url "https://github.com/m2ym/popup-el.git"
+               :features popup)
+        (:name birds-of-paradise-plus-theme
                :type git
                :url "git://github.com/jimeh/birds-of-paradise-plus-theme.el.git"
                :post-init (lambda ()