Hi there,
OK, from what you've said so far you are using Live Meeting 2005. While you've given quite a bit of detail, it would be good to know the following:
1. Are you a direct cutomer of Microsoft (through a corporate agreement) or are you using the service through a partner?
The reason I ask this is that one reason why you may not have the app sharing function available is because of a change that might have been made by the administrator of the conference centre you use. If you are a corporate customer of Microsoft (directly) then you could speak with whoever is the administrator of the site within your company and check whether they have changed the user policies? Otherwise, if you are a customer of a partner, then you may want to check with them to see if they have changed something.
2. Re-install. While not something that we like doing, it should be worhtwhile uninstalling the client (I'm assuming that you have the rights to do this, if not you need to speak with your intenral support folks). Then don't re-install, just go to a meeting you have created and let Live Meeting push down a new client to you. But again, check with your support folks before doing this as they may be avaiable to do this, or you simply may not be allowed to do this as part of the company policy.
Try again, using LM. If you are still having problems then contact support through http://www.livemeeting.com/ask
Thanks and good luck
Simon