Try this instead...
Make sure you have at least 2 points invested in the alchemy skill.. only 1 point wont allow you to make your own potions YET
Level 1 alchemy - You can combine 2 potions to make another (i.e. drag blue over red and make white)
Level 2 alchemy - You can make SMALL potions (Drag a small empty flask from your inventory onto the mushrooms / herbs and the flask will fill up with whatever potion you have just made)
Level 3 alchemy - You can make medium potions (same method, but use a medium flask)
Level 4 alchemy - you can make large potions (same method, but use a large flask)
Level 5 alchemy - You can augment potions (make existing potions larger, by draging them out of the invetory onto an Augment herb)
hope this helps.