Hint #1
If you did it like this:

1 + 2 + 3 + 4 + 5 + 6 + 7

That wouldn’t work because that sums up to 28.

Hint #2
And if you did it like this:

12 + 34 + 56 + 7

That also would not work because that sums up to 109.

Hint #3
There’s actually two correct answers.  Each starts with “1+”.  Can you figure them out?
Answer
1 + 23 + 4 + 5 + 67 = 100

OR

1 + 2 + 34 + 56 + 7 = 100