Problem uploading media

I’m trying to upload a picture on my post, but it seems like an imposible task…
The message I’m getting is: “Unable to create directory wp-content/uploads/2013/05. Is its parent directory writable by the server?”

I’m using WP 3.5.1

This is the information I have on media settings (I had to install the WP Original Media Path so as to get these options)
Uploading Files
Store uploads in this folder wp-content/uploads
Full URL path to files http://bengasi.com.ar/wp-content/uploads
(X) Organize my uploads into month- and year-based folders

I read a lot of posts but can’t seem to fix this…

I didn’t understand about changing permissions, I use a mac.
Please help!

Thanks a lot!