When using neutral densitly (ND) filters is is neccessary to adjust your normal exposure to compensate for the number of stops your ND filter reduces the light transmission.
Download this table for your reference:
Pentax K-5 users the camera metering work for me with a Hoya NDX400 which actual surprised me.
For more information I recommend the following websites:
- The Ultimate Guide To Neutral Density Filters
- Long Exposure Creativity using a 10 stop ND filter...
- Neutral density filter - From Wikipedia, the free encyclopedia