John.Y,
You can reach Mai Po car park by taking a red minibus no. 17 from Yuen Long or Sheung Shui, get off at Mai Po then walk along the Tam Kong Chau Road (about 20 mins). Or simply you can take a taxi from Yuen Long or Sheung Shui if you don't want to walk along the Tam Kong Chau Road.
Please refer to our website for more details:
http://www.hkbws.org.hk/web/eng/new_territories_eng.htm#8
Vicky