For the mobile app, you can find it on the Apple App Store or Android Play Store. As for the Desktop App, it's just a wrapper of the web app, so you don't really need it for development purposes. It is mostly used by enterprise customers for bulk deployment.

On the developer sandbox, it is a shared pod and you will not have access to the admin console. If you are referring to your own company's pod, you will have to ask your colleagues who are the pod administrators.

Perhaps you could describe what you are trying to achieve?

I'm the own who created the account, Normally, I supposed have access to the admin console.
If not What can I do ?

Again, it is not clear which environment you are referring to. Can you please be specific on what you are trying to achieve? You do not require admin console access to use the developer sandbox.