is it a tr 707 that you have? not one of those new groove box things (i'm not even sure if there is a 707 groove box thing)

If it is you should be able to use the din sync out on the 707 to go to the din sync in on the 303.

When you start the 707 it should trigger the 303 and play your patterns

i know the din sync leads are differnt than midi leads but i'm not sure if this makes a differnece