Can someone help me and explain

Im new and looking for a coorelation between computer power in MH/s versus BTC earnings / mining

A formula for calculating the average BTC earnings per hour, at a given difficulty, reward per block, and hash rate is:

bitcoins per hour = ((reward per block)*3600)/((difficulty* 2^32)/(hashes per second))

The current reward per block is 50. That will change in a year or so by being halved. The current difficulty is 434883. Note that in that formula your hash rate is calculated as hashes per second and not megahashes per second. The conversion is 1 megahash/s equals 10^6 hashes/s.

How many MH/s / day = a BTC approx? ie how many MH/s processing power do I need to earn 1 BTC mining

I take it you want to know what your hash rate would need to be in order to earn 1 bitcoin in a 24 hour period. I'm sure you could figure it out from the above formula, but here is a formula that will do the trick:

hashes per second = (difficulty * 2^32)/((reward per block * 3600)/(bitcoins per day/24))

Again, the result is in hashes per second. To convert it to megahashes per second you need to divide the result by 10^6. Plugging in the current difficulty and reward per block, and assuming an average of 1 bitcoin per day, you would need 432.36 Mhash/s.

Difficulty? is it changed on a schedule requiring more MH/s to mine BTC?

The difficulty changes every 2016 blocks such that, on average, 6 blocks are solved every hour. If during the course of a difficulty period more than 6 blocks are solved per hour, then the difficulty increases. If less than 6 blocks are solved per hour, then the difficulty increases. If 6 blocks are solved per hour, then solving 2016 blocks will take 2 weeks. Difficulty periods of 2016 blocks have typically lasted less than 2 weeks because the computational power of the bitcoin network has been consistently increasing over time such that more than 6 blocks are typically solved per hour.