K I messed up your code and made it tidier and stuff but there seems to be some kind of bug that stops it sliding all the way:
sync on
sync rate 60
backdrop on
make object sphere 1,100
position object 1,-100,0,0
make object cube 2,100
position object 2,100,0,0
zrotate object 2,45
move object up 2,45
sc_setupobject 2,0,0
OLDX=-100
OLDY=0
X=-100
Y=0
do
OLDX=object position x(1) : OLDY=object position y(1)
move object right 1,rightkey()-leftkey()
X=object position x(1) : Y=object position y(1)
if sc_sphereslide(2,OLDX,OLDY,0,X,Y,0,50,0)>0
position object 1,sc_getcollisionslidex(),sc_getcollisionslidey(),sc_getcollisionslidez()
endif
sync
loop
delete memblock 1
Probably best to ask Paul about it (seeing as he made the plugin xD)
EDIT:
Ah floats, that's it. xD
I knew something felt wrong when I was coding that.

!