FLC OF ACR MDT ACFT INADVERTENTLY DEVIATED FROM ASSIGNED TRACK DUE TO MISHANDLING THE AUTOPLT FLT DIRECTOR SETTINGS.

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THIS EVENT I CONSIDER TO BE A NAV DIFFICULTY. OUR RNAV WAS SET UP INCORRECTLY FOR LNAV USING THE AUTOPLT. AFTER DEPARTING LGA WE RECEIVED RADAR VECTORS TO INTERCEPT V99 (LGA 055 DEG RADIAL). UPON SELECTING LNAV ON THE RNAV; THE ACFT HUNTED FOR THE RADIAL BY TURNING L THEN R. CREW RESELECTED HDG MODE TO INTERCEPT THE RADIAL. ATC THEN CLRED US DIRECT TO BAYYS INTXN. INTERCEPT WAS MADE USING THE HDG MODE AGAIN THEN SWITCHED TO LNAV. THE CAPT; WHILE TRYING TO DAMPEN THE INITIAL ACFT WANDERINGS; SELECTED OBS MODE ON THE RNAV WITHOUT INFORMING THE FO; WHO WAS THE PF. (A NORMAL RNAV SETTING WOULD BE AUTO/LEG). AUTOPLT WAS ENGAGED WHEN ESTABLISHED INBOUND TO BAYYS. BAYYS AND SEALL INTXNS LIE ON THE SAME RADIAL (BDR 054 DEG RADIAL) 8 MI APART. THE 290 DEG RADIAL OFF THE GROTON VOR WAS BEING USED TO BACKUP THE RNAV; WHICH ALSO DEFINES SEALL INTXN ON THE #2 NAV RADIO. AFTER XING SEALL; THE ACFT DID NOT TRACK INBOUND TO GROTON. AFTER NOTICING THE ERROR; CREW CALLED ATC FOR A VECTOR TO GROTON VOR. NO FURTHER DIFFICULTIES WERE EXPERIENCED. CONTRIBUTING FACTORS INCLUDED: CREW DISTR WITH ATIS; CREW BEING RUSHED WITH AN ACFT SWAP AT LGA. FO WAS INEXPERIENCED WITH RNAV USE. CAPT WAS ALSO PREOCCUPIED MENTALLY BECAUSE HE HAD BEEN INFORMED OF HIS BASE BEING CLOSED AND HE BEING RELOCATED ABOUT 1 HR EARLIER.