I have my HI 1746WS in a local SLC-500 rack but not getting any weight.

I have my HI 1746WS in a local SLC-500 rack using RSLOGIX 500.  I have set up the card configuration for 32 scanned input and output words with the M0 and M1 length of 128.  
From your example program "1746 local example.rss", subroutine 3, I copied the 3 rungs to read the gross weight.  It's not reading anything.  The one scale is reading zero and the other is reading a negative number.  The I:25.18 bits are all 1.  What am I doing wrong?

It looks like you have the module in slot 25 based on your address I:25.18 for your weight.

Since you have all 1's in word 18, it sounds like you may have a negative weight reading.  Realize that the weight value is a 32 bit value so word 18 is only half the value, the most significant 16 bits of the weight, and the least significant 16 bits will be in word 19.  If your multiplier is set to a value other than zero, then your weight will be in integer format and negative weights will show up as: word 18 all 1's and word 19 shows your weight.

Remember that the Multiplier determines the format the weight will be in.  If it is zero, then the weight will be in IEEE float format.  You would need to copy words 18 & 19 into a float register to read the weight correctly.
If the Multiplier is a non-zero value, then the weight will be multiplied by the Multiplier value and transferred as an integer.  Again, keep in mind that the value is split between words 18 & 19 with 18 containing the Most significant 16 bits of the weight, and the least significant 16 bits will be in word 19.