Hi I've got a Yamaha P-150 el.piano and recently I've bought a Roland JV-1010 which I want to use together with the P-150. I've tried to connect them, but the P-150 won't send MIDI-signals. If someone have a solution to my problem I'd be most grateful.

Two possibilities to consider before deciding your piano doesn't send: First make sure both are on the same 'base channel' or that the JV is set to Omni; second, the JV probably has a MIDI filter - you can select types of messages to be acknowledged or ignored. Make sure they're all enabled.

In short: learn all about the Master or Global settings on all your gear.
mn