I have seen this part on a me 262 model kit but I don't know what it is... I've make some research on it but I find nothing! Maybe it can be radar, or a weapon? Do you know what is this? And also its function?
They are Werfer-Granate 21 rocket launchers -
The Werfer-Granate 21 rocket launcher, also known as the BR 21 (the "BR" standing for Bordrakete) in official Luftwaffe manuals, was a weapon used by the German Luftwaffe during World War II and was the first on-board rocket placed into service by the Luftwaffe, first introduced in mid 1943.
On the Me262 they were mounted one each side of the nosegear well.
This is the exact aircraft your model is based on -
Messerschmitt Me262A-1a 'Green 3' of the Geschwaderstab [headquarters unit] of JG 7 prepared to move off across the concrete surface at Brandenburg-Briest in February or early March 1945. The aircraft, finished in a relatively rare application of streaked horizontal lines, has been fitted with a pair of 21cm WGr air-to-air mortar tubes visible beneath the fuselage aft of the nose wheel. (source)