This question can be answered best by going over an example.
Let's assume the user would like to implement a 64-bit LFSR with polynomial X^64+X^4+X^3+X^1+1.
Excluding the + 1 at the end of the polynomial above, user should create a binary string that represents each coefficient (1 or 0 depending on whether it is present in the equation) for each of X^1 through X^64:
X^64..............................................................X^1
1000000000000000000000000000000000000000000000000000000000001101
Then, convert this binary string to hexadecimal: 800000000000000D
User should now enter this hexadecimal value into the Feedback Polynomial field.