Just got a new Isis BB. Its marked 68/73 mm to go in a 68 mm BB shell. It has 1x 2.5mm spacer. Do I need to use it, on the drive side, to in effect make my BB shell 78mm or is it to make the BB fit a 73mm BB shell?

Use it on the drive side assuming you are not using a BB mounted chain device.

Edit: (3 spacers would be required for a 68mm BB.)

If you aren’t running any other BB mounted chain devices or mechs, then for a 68mm shell you need one spacer non drive side, and two spacers drive side. It should have come with three. If it was new get back to the seller, if it was secondhand send me your address and I’ll get some off to you in the the post.
Or, having just re-read your post, are you fitting it in a 73mm shell? in which case you just need the one spacer drive side, unless you have a BB mounted device.

If its an Isis bb not a gxp then it will be either 68 or 73 mm shell and the 2.5mm spacer is to allow for a bob mounted front mech. Is it Isis or gxp?

SRAM / truvativ Isis bbs used to come with just one spacer and would fit 68/73 mm shells

See here.
http://cdn.sram.com/cdn/farfuture/-YO8l_0NkMM3ol2Jf-2hBfsRFKmWrEnOLvIPCEzc93c/mtime:1372788172/sites/default/files/techdocs/bb-e.pdf

The three spacers is for ht2 not gxp or Isis

And yes despite what’s above the same bb was and is sold as being for both shell widths despite only having one spacer

Just don’t bottom the left hand cup onto the frame, it may have a rubber o ring to try and stop you going to far anyway

ISIS
New from a shop. 68mm shell
1 spacer

As long as you have not got a chain device or mech held on by the bb
Put the spacer on the drive side. Just don’t go crazy with the torque on the lh cup

What orangeboy says.