For questions like this, in general, you really should give examples of what you're attempting so we can see if there's an issue in your code.
In this case, I'll assume you're putting the character entity references in the correct place and the problem is simply that you're not using valid references. Assuming you want the upwards and downwards arrows, they are as follows (entity, decimal, hexadecimal):
<!-- upwards arrow -->
↑
↑
↑
<!-- downwards arrow -->
↓
↓
↓
TL;DR: You're probably just using invalid entity references—one too few "r".