If you get a PEAR_Error (or a MDB2_Error object), try using getMessage() and getUserInfo(). They do often give you more information about the cause of the error.
If you get this error after creating the MDB2 instance it means that you don't have any MDB2 database driver installed. Since most people use only one database system, it is unnecessary to install 15 driver files.
If SQLite is the database of your choice, do a pear install MDB2_Driver_sqlite or simply pear install MDB2#sqlite and it should work.
Also search for MDB2 drivers in the PEAR package list.
If you get this error when trying to install a driver it means that the php.ini loaded in the given installer does not see the "XXX" extension. Either you forgot to install the extension all together or you need to make sure that the extension is actived in all relevant php.ini files. Note that there are usually separate php.ini files for the CLI and your other SAPIs.
If all else failes do pear install -nodeps MDB2_Driver_XXX and it should work.
If you work with more than one database at the same time, you may notice that as soon you connect to the second db the connection to the first one is lost, since the DBMS reuses the last open connection link. To solve the problem, just set the new_link DSN option to TRUE.