Here is the raw data:

Range---Cat3---Cat2
0 -5------2------5
6-11------3------6
12-25-----5-----10
26-50-----8-----15
51-100---12-----20
100+-----15-----30

Ex:1
there will be an input box A2. when the user enters the number the formula will check if its in the correct range and set a high and a low range for that number for each catagory (Cat3,Cat2) Example user enters an 8 in A1.

There are two catagories and a high and a low range.

A3 range for catagory 3 (B3) low= 5 (8-3) (C3) high= 11 (8+3)
A4 range for catagory 2 (B4) low = 2 (8-6) (C4) high = 14 (8+6)

Ex:2
If user enters 80 in A2
B3 = 68
C3= 92
B4 = 60
C4 = 100

I have tried several formula's and I can't get anything to work right.

You need to divide your range column into two columns - one with the lower bound, and one one with the upper. The upper column will just be for reference. Also, replace "100+" with "101" in the first column. Then you'll be able to use the table in VLOOKUP formulae:
For B3 - "=A2 - VLOOKUP(A2, [Range], 3, True)"
For C3 - "=A2 + VLOOKUP(A2, [Range], 3, True)"
For B3 - "=A2 - VLOOKUP(A2, [Range], 4, True)"
For B4 - "=A2 + VLOOKUP(A2, [Range], 4, True)"