1. Mount the MicroSD card via the phone and create a folder called "ringtone" then drag your files into it. There shouldn't be any issues if they're regular MP3's without any DRM.
2. By default the stock option "android system" doesn't allow you to choose anything else other than the default system sounds.
3. I'd suggest you download tone picker (it's free) from the market and using it to set your ringtones.
4. Open Settings goto Sounds & Display and choose Phone ringtone from the list, a pop-up should appear, now choose tone picker and tone picker again. Now, there should be a another pop-up with all the usable files. Choose what you want and click ok.
WIND.GR T-Mobile.DE E-Plus Tchibo Aldi K&K MagnaMobile.DE
On your sd card make a folder called audio, then in that folder make 3 more folders alarms, notifications and ringtones. Then place the ringtones you want in the ringtone directory and you may now select them. I do not remember where I found this but it works. I simply copy my ringtones to that directory and I am able to access them