Clicking in the small round button at the upper right of the calculator makes the four memory registers visible to the right of the working registers. Clicking in a memory register will hide that memory and all above it.
Click on [Sto] and then a number key from 1 to 4 to put x into the memory corresponding to the number clicked. The [Rcl] key, followed by a number key, puts the memory corresponding to the number into register x, lifting the stack if called for.
The [Sto+], followed by a number key, key adds x to the memory value.
The [ClM] key (don't follow with a number) clears ALL memory registers.