Once you're logged in to the Pi, check to see if the Pi recognizes the USB device using the following command:
lsusb
You should see the following:
Bus 001 Device 004: ID 7392:7811 Edimax Technology Co., Ltd EW-7811Un 802.11n Wireless Adapter [Realtek RTL8188CUS]
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. SMSC9512/9514 Fast Ethernet Adapter
Bus 001 Device 002: ID 0424:9514 Standard Microsystems Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Notice the first line.
Now we'll check to see if the device drivers are loaded. To list the kernal modules use:
lsmod
You should see something like:
Module Size Used by
cfg80211
rfkill
8192cu
bcm2835_gpiomem
...
8192cu is what we're looking for, and it looks like it's installed.
As a final check, run:
iwconfig
And you should see the wireless adapter here:
wlan0 unassociated Nickname:"<WIFI@REALTEK>"
Mode:Managed Frequency:2.462 GHz Access Point: 20:3D:66:44:C6:70
Bit Rate:72.2 Mb/s Sensitivity:0/0
Retry:off RTS thr:off Fragment thr:off
Power Management:off
Link Quality=100/100 Signal level=100/100 Noise level=0/100
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0