I have a magicloader aluminum trailer for my 32 foot boat . It has the tension style hydraulic brakes on it , i believe it has the electric solonoid on it as well. The guy that bought it new , appeared to have just clipped the large plug in off of it ,and wired the 5 pin plug in onto it . The brake fluid resevoir appears to have never had fluid in it what so ever . Question is , do i just put the correct round plug in back on and fill the resevoir with fluid and bam now i have brakes? Any help appreciated...

The short answer is no. Consider finding a good local brake mechanic or shop that can help you with these brake issues. This system is too important for trial and error. There is nothing special about many/most boat trailer brake systems, most good local mechanics can help you. Be safe...

just fill it with fluid.

Your brake system is called "surge brakes".

Depends on how much you can do yourself, but I would rewire the trailer->truck connector and fill the reservoir with fluid.

BUT, you will need to flush/bleed the brakes out individually.
You should see a bleeder screw on each of the brakes. You need to bleed them from the farthest one from the coupler, forward. Look at your lines and figure out which one of the brakes is plumbed the farthest from the surge brake coupler, and work your way forward.

If this does not get your brake system working, take it to a professional trailer shop for a complete refresh.

Whats the advantage to using a round plug (7 pin?) instead of the flat 5 pin that it sounds like you have.

I am fairly mechanical i guess . I changed the oil once in my lawnmower

if the trailer does not have electric brakes a 7 way connector is overkill. the 5 way will do what you need it to. if the fluid is jusdt low, adding to it should be all you need to do. if there is a leak, it will have tol be fixed then bled. start with the simpliest thing first, add fluid.

Honestly , thank you everyone for the advice . I have had normal electric breaks on other trailers ,just not the surge breaks. I didnt know what the extra wire was for that went to the solonoid on top of the tounge . But im pretty sure its the wire that gets hooked to the reverse light , to free the brakes when backing , when all is functioning of course.
It appears that the resevoir has never had fluid in it . EVER.
So i wanted to make sure i got some advice before moving forward on somthing i knew nothing about . Looks pretty cut and dry though ,i will keep all informed.

the 5th wire should be blue, it is the one that activates the solenoid to allow you to back the trailer up.

Fill it up and it should be fine. Unless it's electric over hydraulic, the 5 way is all you need.