seems easy enough...
1) place foot on and depress brake pedal
2) depress shift lever buttion
3) move selector to gear position you wish to use

once the selector is in the OverDrive position, is it supposed to lock in that detent?
i found out accidently yesterday that mine does not stay in OverDrive by bumping the selector out of OverDrive into Neutral with my right elbow while turning my body to the left to check traffic i'm just glad i was at a stop at the time.

all the other floor shift cars i have driven lock in once you select Drive is mine out of adjustment or am i paranoid?

you're paranoid...every car, bonneville or not, should be able to bump from OD into neutral without pressing the button...also, if you shift to the last gear "1", you should be able to shift all the way up to neutral without the button, and the same going back from reverse, you should be able to slide it from reverse to neutral or overdrive without the button

As Nick mentions..almost every car is setup this way. However some aren't. To my knowledge all Bonne'* are this way.

Completely normal.