Do you want the character to be able to face the opposite direction he's in while sliding? If that's the case, you'll have to do things a little differently. Instead of checking the X speed, check which key is currently depressed. If the right arrow is down, set the angle to 0, if the left arrow is down, set the angle to 180.
For the sliding, you'll need to check to make sure that neither right nor left is down. You can do that by negating the condition, which you do by right clicking a condition and clicking "Invert condition". As a subevent to that, check if the player is currently walking -- the platform behavior actually has a condition for this. If it is, set the player's animation to the sliding animation.
I'm probably explaining this poorly, so here's a CAP file showing what I'm talking about: Sliding
As for the "Or" condition, you can sandwich it between two conditions. If either of them equate to true, you're event will trigger.