Before exporting your work in Blender, select all (press 'A' key) while in object mode, then import a valid skeleton.nif (usually in your meshes\character\_male subfolder). Set the option: import skeleton only + parent selected. After that, follow the instructions from razorpony. Keep in mind that unless you need to resize, rotate, scale or merge objects and meshes, many adjustments can be done without Blender, using only Nifskope.