there are many good instructables on how to build a pen holder for your moleskine notebook.
I'd like to propose another simple simple simple solution, to carry your pen with your moleskine without building anything.
This solution does not require you to modify your notebook. All you need is... well... a pen.
let's get started...
Step 1: Clip That Thread!
Open your notebook and catch the cloth bookmark with the pen clip. Be sure to keep the clip upwards.
Step 2: Move It Up!
mmmh... move the pen up, on the upper boder of the notebook. Clip, up.
Step 3: Pull!
close the notebook and pull down the bookmark. The left side of the pen will get trapped up there...
Step 4: Block It!
last step: close the notebook with the elastic band as you always do, blocking the pen.
now you can carry your pen with your moleskine wherever you go. easy, isn't it?