Although it's great want to associate yourself with an agency, it's even more important to build a positive, noticeable reputation on oDesk. Here are some videos that will guide you on how to establish yourself fully on oDesk, how to find the right kind of administrative jobs and then the best way to apply!
Create an oDesk Freelancer Profile
Setting a Job Feed on oDesk
Submitting an oDesk Application and Cover Letter
Perhaps by having a professional, established freelancing profile with great feedback then just maybe you'll be noticed by groups and agencies!