Download Skylord's JAVA Enabler to enable DES. If you have original Siemens cable you'll need to read the forum to find out what to do with it to be able to run JAVA Enabler. If you have a non-original cable, you just connect the phone and run the program.
After that you download the Data Exhange Software (DES) from Siemens' website (it's originally for SL45 phone, but after Skylord's program has run this won't be a problem
With this, you can upload midi files up to 64Kbytes, and JAVA programs up to 376 Kbytes (C55 has this much memory)
Hope I could help!