http://www.chiark.greenend.org.uk/~sgtatham/putty/
Get Putty (or SecureCRT or any number of other SSH compliant terminal programs)
log in using the user/pass info and then you should get a prompt
if you have uploaded a file with the crontab entry that you need, you can do:
# get the current crontab listings appended to your file
crontab -l >> newcrontabfilethatyouuploaded
crontab newcrontabfilethatyouuploaded
If you feel comfortable with your editor on there, you could do:
# set the editor to nano just in case
EDITOR=nano;export EDITOR
VISUAL=nano;export VISUAL
crontab -e
and paste the line in.
Then if the editor is nano, control-X will save. If you are in nvi/vi, then inserting the line will be a little challenging if you don't feel comfortable with vi.