I haven't been able to pin down a female friend here since I joined.

First, fill in your profile with honest details about who you are.
Potential friends will want to check that out to see if there's any chance of compatibility. Include details like what you do, your interests and values, and what you think contributes towards a good friendship.

Think of the kind of person you seek.
Then ask questions which have a chance of attracting that kind of person.

Be fairly active here whenever you have a bit of spare time - answering questions that show who you are as a person.

Watch the answers and see whose views are attractive to you.

When you make an approach, be honest. Say what caught your attention. Say you'd like to get to know her better and ask if she would like the same.

Don't worry about the rejections. If she says no then she's the wrong person for you. The right one will come along eventually.