Not able to add ether in my private network's account

Question

1. I have created a small ethereum private network. I opened launched the private network from terminal 1, launched the geth console from terminal 2, and opened mist browser in terminal 3. Once mist is launched, I created a new account (Eherbase) which has currently 0 ether. Now, when I do miner.start(), it gives null and "INFO [03-04|10:49:12] Commit new mining work number=1 txs=0 uncles=0 elapsed=777.632µs", on other console, but the balance for the account is still zero. How can I add ether to my account?

2. I started again with new genesis.json file with a account already having some ether, but I am now not able to see that account in my mist browser? what should I do?

OS - Ubuntu 14.04
geth- 1.8.1-stable-1e67410e
mist - 0.8.10

commands that I used: terminal 1-
Created private network # geth --datadir=./chaindata/ init genesis.json
launch network # geth --port "35555" --datadir=./chaindata/
terminal 2-
open geth console # geth attach ipc:/address_to_geth.ipc
terminal 3-
open mist # mist --rpc /address_to_geth.ipc

genesis.json file

{

"config": {

"chainId": 15, 

"homesteadBlock": 0,

"eip155Block": 0,

"eip158Block": 0

},

"difficulty": "0x20",

"gasLimit": "2100000",

"alloc": {

"7df9a875a174b3bc565e6424a0050ebc1b2d1d82": 
    { "balance": "300000" },

"f41c74c9ae680c1aa78f42e5647a62f353b7bdde": 
    { "balance": "400000" }

}

}