First try to resolve the errors in the console before you join.
18:19:59 [sEVERE] * ! * ! * ! * ! * ! * ! * ! * ! * ! * ! * ! * ! *
18:19:59 [sEVERE] * ! * De Bukkit versie is niet de aangeraden build voor Essent
ials.
18:19:59 [sEVERE] * ! * You need atleast build 2122 of CraftBukkit, download it
from [url]http://dl.bukkit.org/downloads/craftbukkit/[/url]
18:19:59 [sEVERE] * ! * ! * ! * ! * ! * ! * ! * ! * ! * ! * ! * ! *
That is telling you that you have too new of a version of Essentials for Tekkit. You need to downgrade it from the 1.2.4 version of Essentials to a 1.1 version of Essentials because Tekkit is not yet 1.2.
Next up is:
18:19:59 [WARNING] Server permissions file permissions.yml is not valid YAML: ma
pping values are not allowed here
in "<reader>", line 3, column 20:
permissions:
^
There is a specific format for a YAML file. Please post your permissions.yml file with formatting.
As for the craftbukkit.jar, in tekkit the jar file is renamed to tekkit.jar: craftbukkit.jar = tekkit.jar