Phone them up and ask. Most walls need instructors for odd shifts and tend to have more work available once you've proved you're not a numpty. Full-time positions in indoor walls are rarely advertised because they rarely exist - most climbing wall staff are part-time or freelance.
You'll of course have an up-to-date logbook and outdoor first aid certificate, and if you don't hold a recognised award such as CWA or SPA (and even if you do), then you'll need evidence of your experience working with groups. If you want centre work and you have few tickets or little experience, ask yourself what other qualities you can offer a potential employer.
There's plenty of threads on the forum from days gone by about starting out in the outdoor/adventurous activities instruction game - use the search page to find them. It's worth asking yourself why you want to do that kind of work though. If you want to get paid to climb, get a job in a bank and use your paid holidays for climbing trips. If you want to help other people to climb while you stand on the ground belaying or watching other people belaying, then get a job as a climbing instructor, and remember you're entering an industry with low pay and often long and unsociable hours. The most personally and financially rewarding jobs in the field tend to be ones that need a large complement of "soft" skills rather than technical climbing ability.