How can I tune a frequency like 121.62 on a comm radio?
As a complete guess, you're trying to tune 121.620, but you should just be tuning 121.625. From AOPA (emphasis mine):


  All aviation communication, VOR, and ILS frequencies are in the 100
  MHz band, so the first number is always "1." The hundredths digit can
  be only 0, 2, 5, or 7; 1xx.x2 implies 1xx.x25; and 1xx.x7 implies
  1xx.x75--when a frequency is assigned, the ending "5" is often
  omitted. Trailing zeros in the hundreds and thousandths positions are
  always omitted in voice transmissions.


The standard frequency spacing in the US is 25kHz.

Omitting the last digit is also described in the FAA's ATC instructions:


  
  The separate digits of the frequency, inserting the word “point” where the decimal point occurs.
  
  
  (a) Omit digits after the second digit to the right of the decimal
  point.


They include this example:


  135.275 MHz “One three five point two seven.”


And although this probably isn't your issue, it's worth noting that some radios tune in 50kHz increments by default, unless you pull out (usually) the tuning knob to enable 25kHz tuning. I've been momentarily confused by that several times in unfamiliar aircraft. I assume this isn't the issue you have, though, since the manual for the A210 says it uses 25kHz spacing.