set Cubase's tempo to your need.
Import a drumloop.
If the loop i.e. shows 3 3/4 length of your cycle length, prepare for lengthening the drumloop.
You're on Cubase 4, isn't it.
Open the audio editor of the loop.
Create a cycle in that editor with a length of the numbers of bars of the drumloop.
(draw along the bar at the uppersite of the audio editor window.
You don't per se have to activate the cycle play symbol, but it can be handy.
Check the small 'Do' menu in the upperleft corner of the editor.
Choose 'fit audio to loop length' or something.
See what happens.
This can be usefull also:
Say you have a loop to your wish, and you wanne have Cubase exactly following it's tempo.
In the same Do menu you can chose 'fit tempo to audio.
But you can also do this by hand: scroll the tempo count of Cubase while whatching the end of the drumloop in the audio editor.
Scroll untill the graph fits exactly the bar end of the time bar.
You may have to zoom in at the end and scroll the tempo numbers behind the comma.
good luck, it easy and fast if you manage to do it once.
I don't have much loops on real kits.
Smoov Grooves by Big Fish Audio could be nice for you.
Take a start here (but there's so much around):
http://www.dy-nas-d.com/samples.htm