You can exchange foreign currency into Japanese Yen at airport and Joyo Bank in the Tsukuba Center Building (see map of central part of Tsukuba City). This bank is closed on Saturday and Sunday and after 15:00 on weekdays.
Usage of credit cards issued outside of Japan is rather limited . You can draw out Japanese Yen from a credit card in the post office but NOT in Japanese banks; they accepts only credit cards issued within Japan.
Post office in the central part of Tsukuba City is open
8:00 - 21:00 Mon - Fri.
9:00 - 19:00 Sat
If you arrive on the week-end, it will be necessary to exchange at Narita airport.