I have Flame Strike(L5) and Cure Serious Wounds(L4) but they dont even have the button to memorize the spell!!
My mage is a standard mage and has other spells of level 4 & 5 and of the same type (necromancy and evocation) but for some reason I cant write these. I dont already have them in my spell book either.
Any ideas?????

You will notice that the spells have a red hue to them, you cannot use them, because they are priest spells (if in doubt about a particular spell, check the manual),
the other way is to try and place them in your quick slot, I beleive as a mage you will not be able to.

In other words, these spells are only really good for making some money, or having your cleric cast them if he is at a low level (clerics do not need to memoorize their spells, and by the time you find flame strike, you most likely already have the spell in your book) so go ahead and sell them off, they're worth more dead than alive.

You can't use those spells because they're cleric spells. As such, your class prohibits you from using them.
You can always look up the spells in the manual if you're curious.