Welcome to Android Central! Sorry, but do you mean you have a Samsung Galaxy S2? And I don't quite understand what you're asking for. When you insert an external SD card into your phone, it should be recognized automatically. If it isn't, there are a few possibilities:
The card is not formatted, or has the wrong format.
The card is faulty, corrupt, or counterfeit.
There is a problem with the system firmware.
Or perhaps you want a file manager to explore the SD card? Try ES File Explorer.