Want to use Time Capsule as a back up AND external storage device-help!

I know there's been a few variations on this question but I can't seem to find the answer that relates to me so probably being dumb but help would be greatly appreciated.

I bought a 500gb Time Capsule yesterday and finally got it set up through AirPort Utility. I think I've done it right-there's a green light and it's going through the motions of backing up although VERY slowly. It's taken 10 hours to back up 20GB. I've used Time Machine before with an external Seagate drive so I know how to access/check those back ups.

I have a dish on the side of my house (on side of a mountain!) that brings the internet signal into a wireless router and then my MacBook accesses through that WIFI network. I plugged the ethernet cable from one of the standard ports out the back of the router into the WAN port on the Time Capsule as part of the setup and that seems to be right in that I can see the TC and surf the net.

So, now I just want to see the Time capsule as a normal external drive on my desktop and move other files to it so they can come off my Macbook completely but the only way icon I can see if the white one that says 'Backup of...' I've copied stuff onto this but I suspect that was the wrong thing to do.

Finally, I only want to TM to use the TC up to a certain max storage so I always have space free on it but can't find settings to allow me to do that.

I'd be really grateful if someone could tell me how to set this up properly!