Your link to the 4S isn't working, but it should have a jst-xh type balance connector and on one side there should be a red wire and on the other there should be a black wire. These are pack pos and neg - using a DVM/MM you will read full pac voltage across those two leads. These should be the pos and neg you use for the alarm

In this balance scheme, there are 5 wires (1 + number of cells) so there should be a 5 wire balance connector on the lipo. It is keyed to properly align pos and neg - just align the key tabs and solder the red and black into the alarm connector such that both the red and black 'continue' across the connector.

If the wires are not correctly color coded, you'll have to use a DVM/MM to figure out which side of the balance connector is pos and neg.