msp430g2, which port should external oscillator be connected to ?

Omri Berg1 said:

I'm wondering where in the code I let it be known that the physical port for the oscillator is 2.6-2.7?

Not in the code. The pin is not selectable, it is fixed in the hardware. Therefore the information is found in the device datasheet.
In general, external oscillators that are used as input to the clock system are to be connected to XIN/XT1IN/XT2IN pins and replace the respective crystals. If used as clock inputs to peripherals with separate external clock input (such as the timers), then the appropriate timer clock pin is to be used (e.g. TACLK). In this case, the pin might be selectable through the port mapping controller.

For details about the clock system and how external oscillators are to be used on software side, see the clock system chapter of the users guide.

In your specific code snippet, the clock system is configures to expect a 32kHz crystal on XT1IN pin. For an external oscillator, the first line should rather read
BCSCTL3 = (BCSCTL3&~XCAP_3)|LFXT1S_3;

Since the power-on default is XCAP_1 (=0x04), performing an OR with XCAP_0 (=0x00) doesn't have any effect and won't clear the default. While LFXT1S_0 (=0x00) is the default, doesn't change anything too even if it were not the default and is wrong, as it selects crystal operation, not digital oscillator input.

Note that 'oscillator' isn't a synonym for 'crystal'. The MSP has an incomplete internal oscillator circuit that requires an external crystal (and either internal or external capacitors that match the crystal requirements). So the external crystal is part of the internal oscillator circuit. And this internal circuit can be bypassed when a complete external oscillator is connected (that means a circuit or chip that produces a digital square-wave clock signal)